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How to pull C4 Corvette diagnostic codes (1984-1994)

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As I mentioned in one of my first posts, the first day we drove the 1985 Corvette home from buying it the check engine light came on. Although the car ran fine I needed to figure out what the problem was and what I might need to do to fix it. Depending on which year you own will dictate which method you use to pull the codes

You don’t need a scanner to get the error codes, just a paperclip or a wire. No need to buy an expensive scanner and some aftermarket ones can give off incorrect data.

The ALDL (Assembly Line Diagnostic Link) connector is just below the steering wheel on the right side down by the accelerator pedal.

Table of Contents

  • Differences between the C4 Corvette years
  • How to manually get the 1984-1994 error codes
  • Clearing the codes
  • 1984-1994 C4 Error codes
  • Conclusion

Differences between the C4 Corvette years

Between 1984 and the end of the 1993 model year, the ALDL connector used 12 pins. From the beginning of the 1994 model year until the end of the 1995 model year, the OBD-1 system was used and the ALDL connector had 16 pins. The 1996 model year had the 16-pin ALDL connectors but used the new OBD-II system. That system requires a more complex scan tool to uncover its error codes.
This particular page is to pull only the 1984-1989 codes. Other pages to come with the 1990-1996 error codes.

How to manually get the 1984-1994 error codes

The 1984 – 1989 C4’s ALDL (diagnostic) readout is enabled by grounding pin “A” to ground. To read the codes, short pins “A” and “B” together with a small piece of wire or paper clip. Grounding pin “A” will enable the diagnostic readout of the ECM/PCM. The 1984-1989 C4s have a ‘Check Engine’ light (early C4s) that flashes Code 12 (a single flash followed by two flashes) three times. Code 12 is a delimiter or marker code that identifies the beginning and end of the error code string. After the three Code 12 flashes, you will receive an error code (or codes) or another string of Code 12 flashes if you don’t have any trouble codes stored.

Every code is repeated three times with a long pause between each code group a 32 or Flash Flash Flash pause followed by 2 flashes, repeated three times, followed by a long pause with any additional codes stored then flashed.

In 1990, the interior and dash changed along with the computer system. The codes are stored in the CCM. The technique for checking codes is similar, but you jump the pins A and G on the connector. The speedometer is used for reading the codes.

Clearing the codes

Remember to remove the shorting device from the connector after you have read the codes.

To clear the codes, remove the negative battery cable for a minimum of 30 seconds. Clearing the codes will clear all stored codes and memory (radio button presets, clock, trip odometer, average gas mileage memory, power seats). Your ECM/CCM computer will also have to relearn timing/mixture/exhaust emissions.

Be sure the ignition is in the off position. If you reconnect the battery with the ignition switch on you can destroy the ECM/CCM module.

1984-1994 C4 Error codes

1984-1991 C4 Error codes

Code 12 – Normal No Codes
Code 13 – Open Oxygen Sensor Circuit
Code 14 – Coolant Sensor Circuit Low
Code 15 – Coolant Sensor Circuit High
Code 21 – Throttle Position Sensor High
Code 22 – Throttle Position Sensor Low
Code 23 – Manifold Air Temperature Circuit High
Code 24 – Vehicle Speed Sensor
Code 25 – Manifold Air Temperature Circuit Low
Code 32 – EGR System Failure
Code 33 – Mass Air Flow Sensor High
Code 34 – Mass Air Flow Sensor Low
Code 36 – Mass Air Flow Sensor Burn-Off Function Fault
Code 41 – Cylinder Select Error
Code 42 – Electronic Spark Timing
Code 43 – Electronic Spark Control
Code 44 – Lean Exhaust indication
Code 45 – Rich Exhaust Indication
Code 46 – Vehicle Anti Theft Fault
Code 51 – Faulty Mem-Cal
Code 52 – Fuel Calpak Missing
Code 52 – Engine Oil Temperature Sensor Low
Code 53 – System Over Voltage
Code 54- Fuel Pump Circuit Low Voltage
Code 55 – Defective ECM
Code 62 – Engine Oil Temperature Sensor Circuit High

1992-1994 C4 Corvette error codes

Code 12 – Normal No Codes
Code 13 – Left Oxygen Sensor Circuit
Code 14 – Coolant Temperature Sensor Circuit High
Code 15 – Coolant Temperature Sensor Circuit Low
Code 16 – Opti-Spark Ignition Timing System ( Low Pulse)
Code 21 – Throttle Position Sensor Circuit High
Code 22 – Throttle Position Sensor Circuit Low
Code 23 – Intake Air Temperature Sensor Circuit Low
Code 24 – Vehicle Speed Sensor Circuit
Code 25 – Intake Temperature Sensor Circuit High
Code 26 – Quad-Driver Module #1 Circuit
Code 27 – Quad-Driver Module #2 Circuit
Code 28 – Quad-Driver Module #3 Circuit
Code 32 – Exhaust Gas Recirculation Circuit
Code 33 – Manifold Absolute Pressure Sensor Circuit Low
Code 34 – Manifold Absolute Pressure Sensor Circuit High
Code 36 – Opti-Spark Ignition Timing System (High Resolution Pulse)
Code 41 – Electronic Spark Timing Circuit Open
Code 42 – Electronic Spark Timing Circuit Grounded
Code 43 – Electronic Spark Control Circuit
Code 44 – Left Oxygen Sensor Circuit Lean
Code 45 – Left Oxygen Sensor Circuit Rich
Code 51 – Mem-Cal Error
Code 52 – Engine Oil Temperature Sensor Circuit Low
Code 53 – System Voltage
Code 55 – Fuel Lean Monitor
Code 56 – Vacuum Sensor Circuit
Code 61 – Secondary Port Throttle Valve System
Code 62 – Engine Oil Temperature Sensor Circuit High
Code 63 – Right Oxygen Sensor Circuit Open
Code 64 – Right Oxygen Sensor Circuit Lean
Code 65 – Right Oxygen Sensor Circuit Rich
Code 66 – A/C Pressure Sensor Circuit Open
Code 67 – A/C Pressure Sensor Circuit (Sensor or A/C Clutch Circuit Problem)
Code 68 – A/C Relay Circuit Shorted
Code 69 – A/C Clutch Circuit
Code 72 – Gear Selector Switch Circuit

Conclusion

Determining your C4 Corvette error codes is not difficult, if you follow these steps. From there you can determine what you need to look at to fix your problem. Sometimes there can be one or several error messages that come up. Try to fix them one at a time if possible. Still, the best method to follow in tracking down these errors is to use the step-by-step processes you can follow in the GM Factory Service Manual. See my review of repair manuals here.

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Pages

  • How to Replace C4 Hydraulic Clutch Master and Slave
  • Where are the correct C4 Corvette jacking points?
  • Best way to bleed your C4 Corvette clutch
  • Changing the front C4 Corvette shocks 1984-87
  • Changing the rear C4 Corvette shocks 1984-87
  • How To Remove and Clean C4 Corvette Valve Covers
  • How to Replace C4 Hydraulic Clutch Master and Slave
  • How to repair a leaking C4 Corvette intake manifold
  • How to repair a C4 Corvette 1984-1987 headlight motor
  • How to pull C4 Corvette diagnostic codes (1984-1994)
  • How to maintain your C4 Corvette Throttle body
  • How to change C4 Corvette Transmission Fluid
  • How To Remove and Clean C4 Corvette Valve Covers
  • What are some C4 Corvette Repairs I have done
  • C4 Corvette Repair Basics for Beginners

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