C4 Diagnostic Trouble Codes

Codes, Codes, Codes…

Have you noticed “SYS” intermittently showing up in your 1990-1996 Corvette’s speedometer display?  This means there is a trouble code stored in one of the on-board control modules.  When the 1990 Corvette debuted, GM introduced ECM (Engine Control Module) and CCM (Central Control Module) trouble codes.   In addition, ABS/ASR (Anti-lock Brake System/Acceleration Slip Regulation) and EBTCM (Electronic Brake Traction Control Module) trouble codes and data were introduced on the 1992-1996 Corvettes.  Although limited ECM data is available, it can point you in the right direction.  For example, the CCM interaction is invaluable when diagnosing interior component or courtesy light problems and the VATS (Vehicle Anti-theft System).  The ALDL (Assembly Line Diagnostic Link) is used to access the codes with a jumper wire.  The jumper wire is placed in the proper connector slots and the interrogation begins.  Interrogation can be a daunting task.

This 1990-1993 ALDL has the jumper in place. Watch that the twisted wires stay inside the terminal during installation. Any loose strands could short another terminal.

1990-1993 Corvettes use the first generation ALDL connector.  It houses a fuel pump override lead that has battery voltage.  This connector position can harm other ALDL circuits if battery voltage is inadvertently jumped to a control circuit.

Our 1994-1996 ALDL has the jumper wire in place ready for CCM interrogation.

1994-1996 Corvettes with the later designated OBDII (On-board Diagnostics) connector use battery voltage for scanner power, so it also has the potential for possible circuit damage. There is no real concern if you are careful during the installation of the jumper wire.  There is no way to damage any component during the interrogation procedures; just turn the ignition key off and start over.


1990-1993 Corvette ECM and CCM codes can be accessed placing a jumper wire in ALDL positions A and G.  Position A is in the top right corner; position G is the first position at the lower left of the ALDL.

1994-1996 Corvette ECM, CCM and ASR/EBTCM codes are accessed placing the jumper wire in ALDL connector positions 4 and 12.  GM used numerals in place of alphas on all 1994 and up Corvettes at the ALDL.  The ALDL numerals start with 1 in the upper left hand position and 9 in the lower left position.  Install one end of the the jumper wire 4 positions from the upper right.  Place the other end of your jumper wire in the position directly below number 4.

The ALDL connector is located above your right knee when seated in the driver seat.

The CCM will display all module codes (sequentially) in the speedometer read-out area.   Module 1 CCM (central control module) is first, followed by module 4 ECM (engine control module) and finishing with module 9 ABS/ASR EBTCM (anti-lock brake system/acceleration slip regulation/electronic brake traction control module) codes. 1990-1991 Corvette CCMs will not display any ABS/ASR or EBTCM codes.

Module numbers 1, 4 or 9 will display directly below the speedo readout area.   The codes themselves will be displayed in place of the speedo readout with a prefix:  H for History or C for Current, followed by the code.  Once all module codes are displayed, “—” displays, ending the sequence.  If any codes are missed, turn the ignition off and then back to the on position to restart the code display.  There is no limit to this procedure.

With LT5 engines, the automatic or manual sequence will not display the ECM codes.  A scanner must be used.  All other codes will be displayed.

Example: H72 code.  This may have been caused from someone driving the car with the radio out for service.  The code set and was not cleared after repairs were done.  If the problem causing the code is fixed, the code must be cleared.  The CCM will not display “SYS” if the problem is repaired.   However, the code will remain in history until cleared.  It’s always best to record all the codes, clear them, and run the engine or operate the system causing the code to see if any codes reoccur.

Once the automatic code read-out sequence is complete, a manual mode is entered.  When a manual mode is entered, the speedometer read-out area will be blank.  The trip monitor area will display “1.0”, indicating module 1 is ready for manual control using the DIC (Driver Information Center) buttons.

Push the “TRIP/ODO” button a defined number of times to select the system you would like to interrogate:

Once and “1.1” will be displayed.  Fault codes could then be displayed.

Twice 1.2 to display 1.2 (CCM data).

Three times to display 1.3 (CCM inputs/status).  This is an interactive display to verify that the CCM is receiving inputs from switches, etc.

Four times to display 1.4.  This is also interactive, as it allows you to actuate some of the CCM control circuits.

CCM data, inputs and status will be displayed in the odometer readout area.

Once you find the mode you want to display, press “ENG/MET” to advance to that particular function.  Press “FUEL INFO” to go backwards.

Example: to determine if the door pin switches are working properly, start by pushing the “TRIP/ODO” button until reaching mode 1.3.  Then, push the “ENG/MET” button until reaching 04 in the 1.3 mode list.  If the door is open, “1” should display in the odometer readout area.  When the door is closed, “0” should then be displayed next to the “1”.  Each time the door is opened and closed, the respective numeral will display.  “1 0 1” will display if the door was closed, opened and closed again.

TIP: There is no danger of damaging any circuits during testing.  Turn the ignition off and restart the automatic sequence to enter manual mode.

Module 1 CCM

Code                  Definition

1.1                      DISPLAY CCM fault codes

12                       On-board diagnostics no codes (this is a good thing: no problem found)

13                       DIC switches open or shorted to battery

14                       DIC switches shorted to ground

16                        Ignition 3 fuse circuit open

21                        Horn relay coil shorted to battery or CCM internal open circuit

22                       Rear defogger relay coil shorted to battery or CCM internal open circuit

24                       Courtesy lamp relay coil shorted to battery or CCM internal open circuit

25                       Courtesy lamp relay coil circuit open or shorted to ground

26                       LCD blanking control circuit shorted to battery or CCM internal open

27                      LCD blanking control circuit open or shorted to ground

31                       LCD data circuit shorted to battery or CCM internal open

32                       LCD data circuit open or shorted to ground

33                      Data clock circuit shorted to battery or CCM internal open circuit

34                      Data clock circuit open or shorted to ground

35                      Data strobe circuit shorted to battery or CCM internal open circuit

36                      Data strobe circuit open or shorted to ground

37                      M clock circuit shorted to battery or CCM internal open circuit

38                      M clock circuit open or shorted to ground

41                      Loss of ECM serial data communications

51                      Pass-key invalid key detection

52                      Pass-key key detection circuit shorted

53                      Pass-key – key detection circuit open or shorted to battery

54                      FEDS fuel enable failure

61                      Pass-key – key #1 programming resistance out of range

62                      Pass-key – key #2 programming resistance out of range

63                      Pass-key – key #2 programming resistance low

71                      LCD dimming output circuit shorted to battery or CCM internal open circuit

72                      LCD dimming output circuit open or shorted to ground

73                      LED display dimming output circuit shorted to battery or CCM internal open circuit

74                      LED display dimming output circuit open or shorted to ground

1.2 DISPLAY CCM data

Code                   Definition

01                        Fuel level (gallons; tenths)

02                        Dimming potentiometer (A/D) counts (see note A)

03                        Ambient light sensor (A/D) counts

04                        Rear defogger timer (seconds)     (see note B)

05                        Vehicle speed (MPH)

06                        Pass-key (A/D) counts

07                        Ignition voltage

08                        Switched battery voltage

09                       Cluster incandescent lamp dimming PWM (0-100%)

10                       Cluster LCD back-light lamps dimming PWM (0-100%)

11                        Radio and climate control LCD back-light lamps dimming PWM (0-100%)

12                        LED dimming PWM (0-100%)

13                       Oil monitor effective revolution (100,000 revolutions per count) (see note C)

14 C                    CM software version (this display is available on 1992 and up Corvettes)


1: headlights or parking lights must be on

2: engine must be running

3: LT1 0/200 counts/LT5 0-250 counts

1.3 DISPLAY CCM inputs/status

Input ID Number    0=disabled, English, off, closed or no

01 Pass-Key fuel                          1=enabled

02 English/metric status                  1=Metric

03 Door key switch alarm info            1=enabled

04 Right door ajar                        1=open

05 Left door ajar                         1=open

06 Key in ignition                        1=yes

07 Hatch ajar                             1=open

08 Power door unlock                      1=yes

09 Power door lock                        1=yes

10 parking lights                         1=”on”

11 Rear defogger input                    1=”on”

12 Seat belt switch                       1=buckled

13 High beam switch input                 1=”on”

14 Low oil level switch                   1=low oil (this is used on 1992+ Corvettes)

1.4 CYCLE CCM outputs

01 Change oil indicator

02 Check gauges indicator   (see note 1)

03 Fasten seatbelt indicator

04 Security lamp indicator

05 High beam indicator

06 Chime 1   (indicated as chime 0 1990-1991 Corvette)

07 Chime 2   (indicated as chime 1 1990-1991 Corvette)

08 LCD blanking control

09 Horn Relay (1990-1991 Corvette) Rear defogger relay (1992-1996 Corvette)

10 Rear defogger relay (1990-1991 Corvette) Courtesy lamp relay (1992-1996 Corvette)

11 Courtesy lamp relay (1990-1991 Corvette) Low oil indicator (1992-1996 Corvette)

12 Not used (1990-1991 Corvette) Starter enable relay (1992-1996 Corvette) (see note 2)

13 Starter enable relay (1990-1991 Corvette) Delayed accessory bus relay (1992-1996 Corvette)

14 Not used (1990-1991 Corvette) Horn relay (1992-1996 Corvette)

Note A: engine must be running to see indicator cycle; it will be on without the engine running

B: the starter enable relay output will be cycled only when the proper Pass-key is in the ignition, or if Pass-key is not programmed.

Clearing the CCM codes can be done after the automatic sequence of codes has occurred with the following instructions:

Press the TRIP RESET button until “1.0” is displayed in the module number area.  Then press the TRIP/ODO button until “1.7” is displayed in the module number area.  Once there, press and hold the ENG/MET button until “—“is displayed.  This will clear any CCM codes.   After “—“ is displayed, turn the key off, wait ten seconds, turn the key back on and check for any CCM codes.

CCM Input and Output Testing

Courtesy light problems are a common issue.  The CCM manual control can be very helpful in diagnosing the problem.  When cycling the courtesy lamps in CCM manual control mode, we can prove that the courtesy lamp relay and all courtesy lamps are functioning properly.  When there is an intermittent courtesy lamp problem, try cycling the control relay repeatedly in an attempt to coerce it into acting up.  If you listen carefully, you can hear the relay click as you cycle the courtesy lamps.  If the relay clicks, the CCM and wiring to the relay are working.  There is a possibility that the relay is clicking and the lamps are not coming on.  Relays use a set of points to make the high amp power connection.  The set of points can become corroded and prevent relaying the power to the lamps.  If the relay does not click, check the relay itself.  In most cases, the relay or possibly wiring from the CCM is to blame.  Wiring issues usually come up replacing the heater core or sound system installation.

Door switch input is also helpful.  Go to CCM inputs 1.3 04 and 05.  Try moving the doors when they are closed to simulate bumps during travel.  If the switches show that the door is open, the courtesy lights will come on unexpectedly.

The DIC ambient light sensor is to the left of the trip monitor above the fuel info button.

TIP:  the CCM uses an ambient light sensor input for courtesy light control.  When the CCM is used to cycle the relay, the ambient light sensor input is not used.  Whether there is enough ambient light available or not, the courtesy lights should cycle on and off during testing.  If all else fails and the courtesy lights will not come on in full daylight, test the ambient light sensor operation.  Use a piece of electrical tape and a flashlight.  Place the tape over the light sensor in the DIC button panel’s upper left corner while monitoring CCM 1.2 data at 03.  You should see approximately 240 counts. Shine the flashlight on the sensor the counts should drop to 0.  We have never seen one of the ambient light sensors fail, but as time goes by, they will.

VATS or Pass-key problems occur in the 1990-1996 Corvettes from time to time.  The CCM has more tolerance for ignition key pellet resistance changes than the 1986-1989 Corvettes, making them more reliable.  The ability to access Pass-key trouble codes helps troubleshoot issues more quickly.  A good example would be CCM trouble code 52 (VATS key circuit is shorted).  The most logical explanation is that the wiring insulation has been damaged, allowing the wires to touch each other.  The steering column may be the culprit.  The wiring could be damaged from either tilting the column or rotating  the key cylinder.  CCM code 51 (invalid key detection) would point to an ignition key resistor pellet that is dirty or damaged.  CCM codes 61-65 indicate that the wrong resistance key was being used during programming.

We have found that when CCM codes 41 or 54 display, it has always been an ECM (Engine Control Module) failure.  The first thought is that the CCM would be the culprit, but to date, we have never seen this happen.  The law of averages shows that CCMs have an excellent track record.  We have only seen one failed CCM (in a 1995 Corvette) in all the years we have worked with them.

CCM (Central Control Module) Codes 72 and 74 usually occur after the radio control head was replaced.  The radio head LED and LCD open circuits are noticed by the CCM, setting the code.  There is a simple fix while the radio head is being replaced.  Resistors and one diode must be installed to alleviate the problem.  A 1K 1/2 watt resistor is installed in the following manner: one side of the resistor must be powered by the radio head C2 connector in position 8 (pink wire).  Connect one end of the resistor to this wire.  This will send the correct voltage signal to the CCM once the other side of the resistor is connected.  At radio head connector C1, locate the gray/black striped wire at position 3.  Connect the other end of the resistor here.  Be careful: the radio head has two gray/black striped wires in connector C1.  The gray/black striped wire at position 4 is the incandescent lamp control.  It will not set a code when left disconnected.  To recap: the resistor will be installed at connector C2 position 8 on one end and connector C1 at position 3 for the other end.

LED dimming circuit

The purple/white striped wire at the radio head connector (C1 position 2) requires a 10 Ohm resistor and diode in series with the pink wire.  The same pink wire from connector C2 position 8 is used for power.  Connect one side of the 10 Ohm resistor to the pink wire.  The diode should be installed in series with the resistor.  Diodes are an electrical check valve allowing current to flow in one direction only.  Diodes have an arrow on their barrel denoting direction of current flow.  The arrow should point away from the resistor towards the purple/white striped wire.  One side of the diode connects to the resistor.  The side with arrow pointing out goes to the purple/white striped wire. This will prevent “SYS” from flashing while you’re enjoying the new sound system.

Module 4 is ECM

4.0 ECM waiting for interrogation instructions

4.1 display ECM fault codes

ECM Fault Code List

1984-1991 (L98: VIN 8 )

Code              Definition

12                    No Distributor Reference Pulse (this code is displayed only when engine is not running)

13                    Oxygen Sensor Open

14                    Coolant Sensor High Temp

15                    Coolant sensor Low Temp

21                     TPS Voltage High

22                     TPS Voltage Low

23                    MAT Sensor Temp Low

24                    Vehicle Speed Sensor

25                    MAT Sensor Temp High

32                    EGR System Error

33                    MAP Sensor Voltage High

34                   MAP Sensor Voltage Low

41                   Cylinder Select Error

42                   EST System Error

43                   ESC System Error

44                   Oxygen Sensor (System Lean)

45                   Oxygen Sensor (System Rich)

46                   VATS Error

51                    Mem-Cal Error

52                   Engine Oil Temp Sensor Low

53                   System Voltage Error

54                   Fuel Pump Voltage Low

62                   Engine Oil Temp Sensor High

ECM Fault Code List

1990-1993(LT5/ZR-1; VIN J)

Code        Definition

13              Left Oxygen Sensor Open

14              Coolant sensor High Temp

15              Coolant Sensor Low Temp

16              DIS Fault Line Error

21              TPS Voltage High

22             TPS Voltage Low

23             MAT Sensor Temp Low

24             Vehicle Speed Sensor

25              MAT Sensor Temp High

31              Cam Sensor Missing or Too Many Pulses

33              MAP Sensor Voltage High

34              MAP Sensor Voltage Low

36              DIS Fault Line/Missing or Extra EST Signal

41              Cylinder Select Error

42              EST System Error

43              ESC System Error

44              Oxygen Sensor (Left Bank Lean)

45             Oxygen Sensor (Right Bank Rich)

46             VATS Error

51              Mem-Cal Error

52              Engine Oil Temp Sensor Low

53             System Voltage Error

54             Fuel Pump Voltage Low

55             Fuel Monitor Lean

56             Port Throttle Vacuum Sensor Low or High

61              Secondary Port Throttle System Error

62             Engine Oil Temp Sensor High

63             Right Oxygen Sensor Circuit Open

64             Right Oxygen Sensor (Right Bank Lean)

65             Right Oxygen Sensor (Right Bank Rich)

1992-1996 (LT-1 VIN P)

DTC Codes 72-99 Pertain to 1994-1996 (LT-1 VIN P)

Code        Definition

DTC 11 –  Malfunction Indicator Lamp (MIL) Circuit

DTC 13 –  Bank 1Left Front) Heated Oxygen} Oxygen Sensor (H02S) #1 Circuit (Open Circuit)

DTC 14 –  Engine Coolant Temperature (ECT) Sensor Circuit (Signal Voltage Low,

High Temperature Indicated)

DTC 15 –  Engine Coolant Temperature (ECT) Sensor Circuit (Signal Voltage High,

Low Temperature Indicated)

DTC 16 –  Distributor Ignition System (Low Resolution Pulse)

DTC 18 – Injector Circuit(s)

DTC 21 –  Throttle Position (TP) Sensor Circuit (Signal Voltage High)

DTC 22 –  Throttle Position (TP) Sensor Circuit (Signal Voltage Low)

DTC 23 –  Intake Air Temperature (IAT) Sensor Circuit (Signal Voltage High,

Low Temperature Indicated)

DTC 24 –  Vehicle Speed Sensor (VSS) Circuit

DTC 25 –  Intake Air Temperature (IAT) Sensor Circuit (Signal Voltage Low,

High Temperature Indicated)

DTC 26 –  Evaporative Emission (EVAP) Canister Purge Solenoid Valve Circuit

DTC 27 –  Exhaust Gas Recirculation (EGR) Vacuum Control Signal Solenoid Valve Circuit

DTC 28 –  Transmission Range (TA) Pressure Switch Assembly Fault

DTC 29 –  Secondary Air Injection (AIR) Pump Circuit

DTC 32 –  Exhaust Gas Recirculation (EGR)

DTC 33 –  Manifold Absolute Pressure (MAP) Sensor Circuit (Signal Voltage High – Low Vacuum)

DTC 34 –  Manifold Absolute Pressure (MAP) Sensor Circuit (Signal Voltage Low – High Vacuum)

DTC 36 –  Distributor Ignition System (Faulty High Resolution Pulse or Extra Low Resolution Pulse Detected)

DTC 37 –  Brake Switch Stuck “ON

DTC 38 –  Brake Switch Stuck “OFF”

DTC 41 –  Ignition Control (IC) Circuit (Open Circuit)

DTC 42 –  Ignition Control (IC) Circuit (Shorted or Grounded Circuit)

DTC 43 –  Knock Sensor (KS) Circuit

DTC 44 –  Bank 1 (Left Front) Heated Oxygen Sensor (HO2S) #1 Circuit (Lean Exhaust Indicated)

DTC 45 –  Bank 1 (Left Front) Heated Oxygen Sensor (HO2S) #1 Circuit (Rich Exhaust Indicated)

DTC 47 –  Knock Sensor (KS) Circuit or Module Missing

DTC 48 –  Mass Air Flow (MAF) Sensor Circuit

DTC 50 –  System Voltage Low

DTC 51 –  EEPROM Programming Error

DTC 52 –  Engine Oil Temperature Sensor Circuit (Signal Voltage High, Low Temperature Indicated)

DTC 53 –  System Voltage High

DTC 55 –  Fuel Lean Monitor

DTC 58 –  Transmission Fluid Temperature (TFT) Sensor Circuit Low (High Temperature Indicated)

DTC 59 –  Transmission Fluid Temperature (TFT) Sensor Circuit High (Low Temperature Indicated)

DTC 62 –  Engine Oil Temperature Sensor Circuit (Signal Voltage Low, High Temperature Indicated)

DTC 63 –  Bank 2 (Right Front) Heated Oxygen Sensor (HO2S) #1 Circuit (Open Circuit)

DTC 64 –  Bank 2 (Right Front) Heated Oxygen Sensor (HO2S) #1 Circuit (Lean Exhaust Indicated)

DTC 65 –  Bank 2 (Right Front) Heated Oxygen Sensor (HO2S) #t Circuit (Rich Exhaust Indicated)

DTC 66 –  A/C Refrigerant Pressure Sensor Circuit (Open or Shorted)

DTC 67 –  A/C Refrigerant Pressure Sensor Circuit (Pressure Sensor or A/C Clutch Circuit Problem)

DTC 68 –  A/C Relay Circuit (Shorted Circuit)

DTC 69 –  A/C Clutch Circuit

DTC 70 –  A/C Clutch Relay Driver Circuit

DTC 72 –  Vehicle Speed Sensor Loss

DTC 73 –  Pressure Control Solenoid (PCS) Circuit (Current Error)

DTC 74 –  Traction Control System (TCS) Circuit Low

DTC 75 –  Transmission System Voltage Low

DTC 77 –  Fan Control PCM Terminal “All”

DTC 78 –  Fan Control PCM Terminal “Al0”

DTC 79 –  Transmission Fluid Over-temp

DTC 81 –  Transmission .2-3 Shift Solenoid Circuit

DTC 82 –  Transmission 1-2 Shift Solenoid Circuit

DTC 83 –  TCC PWM Solenoid Circuit Fault

DTC 84 –  3-2 Control Solenoid Circuit (Automatic Transmission)

DTC 84 –  2nd and 3rd Gear Block-out Relay Control Circuit (Manual Transmission)

DTC 85 –  Transmission TCC Stuck “ON”

DTC 90 –  Transmission TCC Solenoid Circuit

DTC 91 –  One to Four Upshift Lamp (Manual Transmission)

DTC 97 –  VSS Output Circuit

DTC 99 –  Tach Output Circuit

DTC P0116 – Engine Coolant Temperature (ECT) Circuit Range/Performance Problem

DTC P0117 – Engine Coolant Temperature (ECT) Circuit Low Input

DTC P0118 – Engine Coolant Temperature (ECT) Circuit High Input

DTC P0131 – Bank 1 HO2S #1 Circuit Low Voltage (Lean

DTC P0132 – Bank 1 HO2S #1 Circuit High Voltage (Rich)

DTC P0133 – Bank 1 HO2S #1 Circuit Slow Response

DTC P0134 – Bank 1 HO2S #1 Circuit No Activity Detected

DTC P0135 – Bank 1 HO2S #1 Heater Circuit

DTC P0151 – Bank 2 HO2S #1 Circuit Low Voltage (Lean)

DTC P0152 – Bank 2 HO2S #1 Circuit High Voltage (Rich)

DTC P0153 – Bank 2 HO2S #1 Circuit Slow Response

DTC P0154 – Bank 2 HO2S #1 Circuit No Activity Detected

DTC P0155 – Bank 2 HO2S #1 Heater Circuit Malfunction

DTC P0158 – Bank 2 HO2S #2 Circuit High Voltage

DTC P0160 – Bank 2 HO2S #2 Circuit No Activity Detected

DTC P0161 – Bank 2 HO2S #2 Heater Circuit Malfunction

DTC P0171 – Bank 1 Fuel Trim System Too Lean

DTC P0172 – Bank 1 Fuel Trim System Too Rich (1 of 2)

DTC P0174 – Bank 2 Fuel Trim System Too Lean

DTC P0175 – Bank 2 Fuel Trim System Too Rich (1 of 2)

DTC PO420 – Bank 2 Catalyst System – Low Oxygen Storage Capacity Indicated

DTC P1114 – Engine Coolant Temperature (ECT) Circuit (intermittent Low Input Voltage)

DTC P1115 – Engine Coolant Temperature (ECT) Circuit (Intermittent High Input Voltage)

DTC P1133 – Bank 1 HO2S #1 Circuit Not Enough Switches

DTC P1153 – Bank 2 HO2S #1 Circuit Not Enough Switches

DTC P1158 – Bank 2 HO2S #2 Shifted Rich

1994-1995 LT5/ZR-1 Trouble codes

Due to federal regulations implementing uniform trouble codes throughout the automotive industry LT5/ZR-1 Corvettes used many of the same 1994 and up codes with the exception of the codes listed below. (These trouble codes may be in addition to the possible codes above or different meaning)

DTC 31 Camshaft Position Sensor Circuit

DTC 41 Cylinder Select Error (Faulty or Incorrect PROM)

DTC 54 Fuel Pump Circuit (Low Voltage)

DTC 56 Secondary Air Inlet Valve Actuator Vacuum Sensor Circuit (Signal Voltage High “High Vacuum or Signal Voltage Low” (Low Vacuum)

DTC 61 Secondary Air Inlet Valve System

DTC 66 Engine Power Mode Switch Circuit (Signal Voltage High or Signal Voltage Low)

4.7 clear ECM fault codes

Press the TRIP RESET button until 4.0 is displayed in the module number area. Then press the TRIP/ODO button until 4.7 is displayed in the module number area.   Press and hold the ENG/MET button until “—“ is displayed. This will clear any ECM codes.   After “—“ is displayed turn the key off, wait ten seconds turn the key back on and check for any ECM codes.


Press the TRIP RESET button until 9.0 is displayed in the module number area.  Press the TRIP/ODO button until “9.7” is displayed in the module number area.  Press and hold the ENG/MET button until “—“ is displayed.  This will clear any ABS/ASR codes. after “—“ is displayed, turn the key off, wait ten seconds, turn the key back on and check for any ABS/ASR codes.

Story and photos courtesy Chris Petris

70 thoughts to “C4 Diagnostic Trouble Codes”

  1. I have a 1995 Corvette and is rarely driven. Started it up today and got a flashing “SYS” It would flask go off and the battery voltage would show. Then come one again. At startup my battery will shoe 14.9 or 15.0 and will go down from there to about 14.4
    Turned off the car and restarted it did not get the “sys”. Cannot find an answer on line

  2. I have 94 starts an runs for 2 sec and shut down haven’t been able to pull any codes with scanner or jumping pins .Speedo shows no information MPH or odometer I’m think may be the ECM any suggestion?

    1. If there are no codes, the first thing to do would be follow the steps in the GM Shop Service manual to check fuel pressure. It could be any number of things including possibly a fuel pump, fuel pump relay, oil pressure issue causing ECM to shut down the fuel pump.

  3. Hi, am in UK with 1990 L98 vert, white/ red interior, has been fine for last two years. Always fires instantly so long as red top Optima charged, car only used in dry, and always garaged. Put it away after last Sunday drive, as usual, now it won’t run, turns over, fires up, dies instantly, “SES” light in DIC flashes every 2 seconds along with all fans start/stop, relays clicking on off, A/C clutch on off in time. What on earth can go wrong when parked up? I have seen “SYS” flash a few times when driving, but didn’t affect the performance, but have ALWAYS been suspicious of low idle oil pressure when hot, faulty oil sensor behind distributor? I know they are a known issue; but impossible to get at, will it be the 3-wire sensor? Has it failed totally on startup, or has the ECM crashed. I am in UK, in darkest west Cornwall, no idea where to get it sorted, I just hope it’s only the oil sensor not seeing 4psi, would very much appreciate your comments, thanks in advance, Cheers from UK.

    1. The first thing to do if you are getting the SYS light is to follow the instructions in this article to check the codes. That may point you in the right direction. The symptoms you are experiencing could be the Oil pressure sensor as you have mentioned, as it sends the signal to the ECM that there is enough oil pressure in the system and it is ok to activate the fuel pump relay etc. We do offer this sensor, part number 304125. It threads into an adapter at the top rear of the engine block.

  4. Harrell Taylor i have a 1995 corvette automatic trans .after driving about 3 miles the trans will not shift into drive 2 or overdrive does not slip at all . I drain the fluid & change filter but still the same problem .Has 90 thousand miles on it OD

  5. I have a ’93 Anniversary Edition. Lately the “Service Eng Soon” light comes on for a minute while driving the car, then turns off. I’m getting codes 1. H74, and 4. H43, 9.0 is —, and 1.0 is blank. Can you elaborate? Thanks!

  6. I have a 1990 corvette conv. I installed a new Pioneer with a harness. Radio plays, but starter cranks and won’t fire up. I think I may have compromised the fuel pulse injectors. I touched a jumper wire from A to G on the central control module, and a code SYS flashed three times. Question: must I keep the jumper wire in to read the codes in order to isolate the problem? And if I obtain an anti-theft bypass switch, would that clear the CCM?

  7. I have a 1996 Corvette. I just finished flushing the cooling system. Now I get an error code that the ARS is disconnected and needs service. What is the ARS? I have a factory Corvette manual, but there is no table of definitions that I can find. Any suggestions?

  8. I have a 1992 C4, I have a problem of air conditioning, the compressor does not start and I have no default display. I tested the compressor it works perfectly because when I switched the relay manually it works. Where would it come. Thank you

    1. Try replacing the a/c pressure clutch cycling switch on the low side aluminum line. Passenger side next to firewall. I just had a C4 with the same problem and the a/c low side pressure clutch cycling switch fixed it.

  9. I have a 1992 LT1. The engine sputters a little when I try to start. After it starts it runs good but the ‘check engine soon’ light comes on. I checked codes but I did not write them down before clearing them, The coolant LCD stays at 142 F. I got some condensation in the interior this winter. Thanks for your time.

  10. I have a 96 corvette. I replaced the ccm and I am now having trouble configuring to the car. I have a tech 1, when I go through the prompts and enter trans, vin, mileage etc. I hit enter to to start and tech 1 comes back ccm download request failed. Company that made new ccm has sent two other computers but all do same thing. I’m lost please help. Have spoke to gm techs, they have no idea and even a couple delco class trainers.

  11. We have a 93 coupe, LT1, that keeps coming up with a SYS error. I jumped from A to G, and noted module 1 shows H65 and H72. I am sure these are history errors. Module 4 says all is well, as is module 9. But what throws me is I get a module 7 error, and all it says is ERR. Can anyone tell me what this means? Car runs great and no other problems.

    1. I’m assuming you put in an aftermarket radio? That’s probably what’s throwing the 72 code. Not sure about the 65 code. There’s a resistor that you can install on the factory plugs on the radio to keep from throwing this code but you’ll have to google it. Hope it helps.

  12. I have a 1993 Corvette. The battery died over the winter even though it was on a trickle charger. It was 7-8 years old when I put in new battery and started car. My odometer reads odo err. Car runs fine, no other lights on.

  13. I have a 1993 corvette LT-1. I have been having intermittent problems with the car trying to stall out, when that happens the ASR comes up on the screen and if it still continues the check engine soon comes up on the screen. Doesn’t happen all the time. The car has 20,000 miles. Other than that the car runs fine.

  14. I have a 1993 Corvette. When you turn the key to start it, a code on the dash by the speedometer says !drakes. The switch will turn back to run position then starts right up, but can’t shut it off and no power to the Windows. You go down the road and turn back, then it’s fine but it’s taking longer and longer. I am afraid it won’t shut down one time . What’s going on with this thing?

  15. I have a 1993 Corvette with 9,000 miles. Last year, the car began to exhibit braking difficulties in that it seemed to take more brake pressure [and for longer duration] to bring the car to a stop. Dealer replaced all rotors, cleaned and inspected brake pads, calipers, and slides, finding no issues. Car still seems to take a lot of brake pressure to stop, takes a long time to stop, and feels like the car is pushing something. A local mechanic used a scan tool and found code “C0072” and cleared it. I have not been able to find the meaning of this code or what ails this car. Any ideas?

  16. In late January a Corvette dealer replaced the horns and relay(s) associated to correct a recurring blown fuse situation. Because of illness, the car has not been driven or started since early February. Yesterday conditions had improved to be point that I wanted to go for a ride. My ’95 Corvette was totally dead. After charging the battery for about 90 minutes, I tried to start it but it would not crank. No Security light is displayed. Key and ignition switch have been cleaned. All fuses check out. Followed instructions on jumping the ALDL. The only code displayed was “H72”. Once “—” displayed, I started a manual query. At “1.1” only “H72” displayed. At 1.2 nothing. At 1.3 “C73” displayed briefly and then the display went dark. After that the only portion of the main display that will light when the switch is in the ON position is “Check Gauges”. Moving to the START position causes PARK and BRAKE PRESSURE to display. The system will no longer display any codes. Again the engine will not crank. Any guidance would be appreciated.

    1. What battery is in the car? Some batteries like Optima once completely dead can be very difficult to bring back to life. Try swapping batteries and confirm if it holds a charge or not.

      1. Battery is an Interstate and is on charger that indicates a full charge. Lights, radio, etc. all work and seem to operate normally.



  19. Ok, I have a 93 C4 automatic w/ 77K. I’ve seen this all over the internet and mine does it also, just randomly quit like it is running out of gas. No ck engine light, other to let you know it stalled. It sometimes starts right back up and other times it takes a while. I’ve replaced the gas cap and fuel pump to eliminate those cheap and quick concerns. It seems to me it mainly does this around 1/2 tank of fuel in it. I jumpered A to G and #1 showed C12 code and #4 showed H15. A to B jumpered showed code 15 (low temp sensor). Any ideas at all?

  20. Have a 93 vette no key in ignition signal and no driver or passenger door ajar signal at ccm and does not crank issue

    1. Code 16 – DIS Fault Line Error. Direct ignition system (DIS), fault line circuit or Distributor ignition system (low resolution pulse) or Missing 2x reference circuit or OPTI-Spark ignition timing system (low resolution pulse) or System voltage out of range.

      1. I just got 93 corvette, it had been setting outside the last 2 yrs. The owner said it runs when it’s hot but not when it’s cold? So I try to start it but no spark. It’s got a new coil, module, ign, S/W, fuel pump. So I wanted to change the key cyl., once inside I found a broken vats wire. Would that have anything to do with the no spark? And why does it still crank over with that a broken vats wire? When I checked for codes, a code 16 came up one time only.

  21. I have a 1991 corvette. The check engine light came on when I was on the highway. I pulled a code32 when I got home. The performance did not change in the car, it ran great. When I fired the engine up again the light was out. I didn’t clear the code. Should I drive until it comes on again or should I change the egr valve again. I’ve done injector,vacuum lines, and egr valve.

    1. Remove the EGR valve and you can clean it. There’s a very loose test you can perform on the EGR valve when it’s off the engine. Shake it. If you hear the valve inside opening and closing, this means that it’s still working and you can probably get away with a cleaning to get things back in order.

  22. I have a 1992 corvette that do not drive much. I just put a new battery in it and now that I drive it occasionally it does like a hick up and wants to stall and the sys and security light start flashing and the speedometer stops working. Also the check engine and the ASR service light comes on. Do you know if that is a battery issue or something else?

    1. My 92 c4 did the same thing, thought it was the battery too. My problem was the slipping if the crank pulley was bad. Installed high tongue performance pulley. A lot of problems went away. Only problem now is the high temp cooling fan relay is bad. Fan stays on even when key out. Could also be the inline fuse behind battery.

  23. My 93 convertible runs fine cold, when it warms up and driving in city traffic, will die while driving. Pull to the side of the road and won’t start for a couple of minutes. Trouble codes showing are H72, H74. I’ve replaced the ICM. Any thoughts?

    1. I would recommend you take your car to a certified technician. When reading codes, H means previous/History and C means Current, followed by the code. Example: H72 code. This may have been caused from someone driving the car with the radio out for service. The code was not cleared after repairs were done. If the problem causing the code is fixed, the code must be cleared. The CCM will not display “SYS” if the problem is repaired.

  24. Hi, so I have a 1992 C4 / LT1 corvette and love her to death, had it around a year and doing some restoration.
    The problem on my vet is once she runs for around 30-45 minutes, starts to runs rough and eventually chokes on me to the point where I have to pull over. When she is cold/start she runs like a beauty and no issues regarding chocking on me. Now I have done some research and pulled the codes from the dashboard, where I placed the short wire into the A & G slot, these are the codes I got.

    C72, H16, H23, H26, H44, H64, H72, H74, Where can I get a list of those codes and attempt to pin point the issues out ?? Lots of H codes ????

    Thx in Advance Bill

    1. H means previous/History and C means Current, followed by the code. Example: H72 code. This may have been caused from someone driving the car with the radio out for service. The code set and was not cleared after repairs were done. If the problem causing the code is fixed, the code must be cleared. The CCM will not display “SYS” if the problem is repaired. However, the code will remain in history until cleared.

  25. Just bought a 93 vette. Having problems with shifting. The service engine light comes on, then goes off, and the green light above the climate off switch starts blinking. What could this be?

  26. 93 C4 LT1 starts and runs for 2 to 3 seconds and cuts out. Ran codes multiple times 1. C41; 4. ERR;9 .71 and 72. Best assessment bad ecm as ccm rarely fails what causes err code in Module 4

    1. Sounds like it could be a security issue. The ECM is looking for a code from the key within a couple of seconds after starting, and if it doesn’t find it, it shuts off the fuel injectors.

  27. I have a 91 coup that will not crank after running about an hour. I have replaced the cap rotor, ICM, coil, fuel pump and fuel filter. I did a diagnostic check and did not see any codes. The car will not start now although it tries to. I am getting a smell of gas too. Any ideas? Also, I spun the motor over with the paper clip still in place by accident. Did I hurt anything?

    1. Take a look at the Start Sequence Flow Chart in this Digital Corvettes Thread. This has step by step information to troubleshoot the no start sequence. The TPS voltage is most likely the next thing to check since you have already done the fuel pump, etc. You should not have hurt anything with the paperclip in place.

  28. I have a very nice 87. However, the exterior temp light and the other lights on the display for a/c, heater etc only come on when the outside temperature is very hot – 80’s and 90’s. Can you tell me where to look to fix this problem? Thanks very much!

  29. Hey there! We have a ’96 model Corvette and the speedometer is playing up, only on cold mornings. It will drop to 0 then up to 80 and all over the place. Seems to have a consistent signal from the trans speed sensor even whilst fault is present. Fine when the day warms up. Possible component failure on a pcb board but I can’t pinpoint it. Have you struck anything like this? Plug and small harness to lcd display tests ok.

  30. I have 94 corvette vin p. No spark after run for 20 mins. Got code 62 . Can this prevent me from starting car? After about 30 min. Cool down, starts up again. Repeat. Thanks

    1. First is it a powertrain or central control module code 62? Powertrain code 62 is engine oil temp circuit signal voltage low (high oil temp indicated), CCM code 62 is incorrect pass-key resistance. The obvious, if the engine oil temp was high enough to indicate overheat so would the coolant and chances are steam would be coming from under the hood. CCM pass-key resistance issues would prevent the engine from cranking and disable fuel injector pulse not ignition. Chances are that the code is not related although it may be pointing in the direction to take the first look. If the code is powertrain 62 the wiring for the engine oil temp sensor may be in bad shape and shorting out. The oil temp sensor wiring located above the oil filter takes a beating from the catalytic converter heat. This is also the location of the PCM grounds which we see damaged from time to time. I would check there first for any broken or loose grounds. If they are okay check to see if you have fuel injector pulse when spark goes away after the 20 minute run. If you are loosing spark and fuel injector pulse it indicates an Opti-Spark distributor or primary ignition system wiring problem.

    2. Doug, I had same problem. I did seal optispark with proper sealant. Must be oil resistant. I had grease on front of engine. Slowing oil to seaport into optispark. Would run fine til oil was thrown around inside,then quit. Then oil would gravitate to the bottom,then allowing it to start 30 min.later. Good luck, Doug P. 94 black rose 300

  31. I replaced 2 back tires on my 1996 Corvette, now the EBS service brake light and ARS light came on. Can you tell me what is going on?

    1. It’s best to first get access to the codes set in the computer with a scan tool before proceeding. That could point directly to a bad ABS sensor/hub assembly.

  32. My 1992 corvette will not start. No fire at plugs. Have replaced the optispark and the water pump that was leaking. New coil wire. Getting code 42, electronic ignition system is grounded. Any suggestions where to look?

    1. Check for injector pulse with a test light or noid light. If you don’t have injector pulse or ignition, I would check wiring from the Opti-Spark to the ECM. If you can’t figure it out after all that then I would assume the ECM is bad.

    2. I had that problem on my 1992. Replaced the spark module next to the coil. Be sure to get a GM one. Bought an after-market one and the car would start and run for a couple seconds then shut down. Too much voltage.

  33. i have a corvette 1991 and the computer send me number c41 and i change my memcal and already send me that and in system 4 send me error and dont let me read anything if you can help me i apreciate alot

    1. Is your C41 a ECM, CCM or ABS code? From the “system 4 sent me an error input” statement I would say that the code was most likely a CCM code. By far we see more C41 Loss of Serial Data CCM codes than C41 Cylinder Select Error ECM codes. Mem-Cal EPROM failures are rare especially if the engine runs. Incorrect EPROM installation, lighting strikes or a major voltage surge like connecting the battery cables incorrectly during jump starts can do it. Although the Factory service Manual will point you to replacing the CCM for the C41 Serial Data Loss we have found it to be an ECM failure each time. I would suggest the you too read the codes via the ALDL and speedo readout method. It starts at diagnostic mode ECM 1 once all the codes are displayed it goes to diagnostic mode CCM 4 displaying codes and finally diagnostic mode 7 for ABS. Record all the codes and then get back for further analysis. Chances are very good that the ECM will require repair.

      http://tech.corvettecentral.com/2011/01/c4-diagnostic-trouble-codes/ has the procedure for reading codes via the speedo

  34. I have a 1990 corvette convertible that I have taken out the factory Bose stereo and am putting in an aftermarket stereo unit. I now have a battery drain issue and from what im hearing there’s a certain fix of unplugging or disconnecting of a few things that im not aware of.Could you possibly tell me what to unhook so that I may fix this problem. thanks for any and all suggestions.

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