GM OBD1 Codes List [Free Download]

Whether you own a classic Chevrolet, Cadillac, or any other GM model, understanding the OBD1 codes specific to your vehicle is crucial for effective troubleshooting and maintenance.

With our complete list of OBD1 codes, you can gain the knowledge and insights needed to decipher GM OBD1 codes, empowering you to take control of your GM car’s health and performance. 

Towards the end of the article, you can perform a self-diagnostic test with or without a scanner, and learn how to read the trouble codes, interpret them, and identify the faulty component or sensor.

Keep reading to find out more!

GM OBD1 Codes List and Definition [FULL]

Free Download: Full GM OBD1 Trouble Codes List and Meaning PDF

Note: Use the search box in the table below to quickly find the specific code you’re looking for.

Code 12No distributor reference pulse (diagnostic test active)
Code 13Oxygen sensor circuit open or no activity
Code 14Engine coolant temp sensor error (high temp indicated)
Code 15Engine coolant temp sensor error (low temp indicated)
Code 16System voltage too low
Code 17Camshaft position sensor error
Code 21Throttle Position sensor error (signal high)
Code 22Throttle position sensor error (signal low)
Code 23Intake air temp sensor error (low temp indicated)
Code 24Vehicle speed sensor error (open circuit or no activity)
Code 25Intake air temp sensor error (high temp indicated)
Code 26Quad Driver Module (QDM A) error
Code 28Auto transmission range pressure switch error
Code 31Waste gate solenoid circuit error
Code 31Gear Position Switch (PRNDL) error
Code 32Baro sensor circuit error/ EGR circuit error/ MAP sensor signal open
Code 33MAP sensor circuit error (signal high indicating low vacuum)
Code 34MAP sensor circuit error (signal low indicating high vacuum)
Code 35IAC problem or idle error
Code 36MAF sensor error/ 24x crankshaft position sensor circuit error/ 4T60-E shifting error
Code 37TCC brake switch circuit error
Code 38TCC brake switch circuit error
Code 39Clutch switch circuit error/ Torque converter clutch error
Code 41Ignition control error/ MEM-CAL error/ Computer Controlled Ignition Cam Sensor Signal error
Code 42Ignition bypass circuit error
Code 43Knock sensor error
Code 44Oxygen sensor error (lean condition indicated)
Code 45Oxygen sensor error (rich condition indicated)
Code 46Pass-Key II (VATS) system error – circuit out of freq range
Code 47UART (serial data) circuit error
Code 51PROM error/ Incorrect MEM-CAL
Code 52Fuel cal-pak incorrect or missing
Code 53System voltage too high
Code 53Digital EGR Valve solenoid 1 error
Code 54Fuel pump circuit low voltage/ Digital EGR Valve solenoid 2 error
Code 55ECM error/Digital EGR Valve solenoid 3 error
Code 56Quad Driver Module (QDM B) error
Code 57Boost Control error
Code 58Trans fluid temp sensor circuit error (low voltage)/ VATS system error
Code 59Trans fluid temp sensor circuit error (high voltage)
Code 61A/C system performance
Code 63Right bank O2 sensor circuit error/ MAP sensor circuit signal voltage high (low vacuum indicated)
Code 64Right bank O2 sensor lean exhaust indicated/ MAP sensor circuit signal voltage low (high vacuum indicated)
Code 65Right bank O2 sensor rich exhaust indicated
Code 66A/C pressure sensor circuit error (low pressure indicated)
Code 67A/C pressure sensor circuit error
Code 68A/C clutch relay circuit error (shorted to ground)
Code 69A/C clutch relay circuit error (open circuit indicated)/ A/C high pressure switch error
Code 70A/C pressure sensor circuit error (high pressure indicated)
Code 71A/C evaporator temp sensor circuit error (low temp indicated)
Code 72VSS signal circuit error
Code 73A/C evaporator temp sensor circuit error (high temp indicated)
Code 75Digital EGR solenoid #1 circuit error
Code 76Digital EGR solenoid #2 circuit error
Code 77Digital EGR solenoid #3 circuit error
Code 79Transmission fluid over temp
Code 80Transmission component slipping
Code 823x Crankshaft position sensor circuit error
Code 85PROM error
Code 86ECM Analog/Digital error
Code 87EEPROM error (flash memory error)
Code 90TCC error
Code 93Transmission pressure control solenoid circuit error
Code 96Transmission system voltage low
Code 98Invalid PCM program
Code 99Invalid PCM program

How to Read OBD1 Codes for GM with a Paper Clip

Do you ever get frustrated with why the check engine light keeps flashing on your vehicle and wanna know what it means? But you don’t have a dedicated scanner to diagnose the issue. 

Don’t worry. I will show you an easy way to read your OBD1 codes using a tiny paper clip. Here are the steps to follow to diagnose the faulty system in your Chevy vehicle:

  1. First, locate the diagnostic plug underneath the dashboard. It should include 12 female pins and has a black plastic casing. 
  2. Unroll a paper clip and fold it into a U or V shape. 
  3. Insert the ends of the unrolled paper clip through pins A and B, which are the two top right pins on the plug. These steps automatically shift the vehicle to diagnostic mode. 
  4.  Next, start the ignition but be careful not to start the engine. 
  5. Fix your eyes on the instrument cluster. Your primary focus should be the check engine light which should start blinking at any moment. 
  6. The diagnostic systems communicate via iterative blinking of the check engine light. Counting the number of blinks should tell you the error code.
  7. The vehicle groups error codes in 2 digits, and each is iterated 3 times. 
  8. The first time the vehicle will blink once, pause, then twice to symbolize code 12. This code means the start of the diagnosis. The code is repeated thrice before trouble codes begin displaying. 
  9. The vehicle will iterate through the error codes, which you should record with a pen and paper, then finish with code 12, indicating the end of the process.
  10. Turn the ignition off and pull out the paper clip before looking up the meaning of the codes in Google. 
A video about how to read OBD1 codes Chevy GMC with paper clip by Man Meets Garage

How to Read OBD1 Codes on GM with a Scanner

Why improvise when you could get a scanner to perform the same function as the paper clip. GM scanners help in reading the error codes without jeopardizing the system’s electronic circuitry. Here is how to use it:

  1. Insert the scanner into the diagnostic plug, which should be located beneath the dashboard. Ensure that it is inserted correctly for its pins to mesh with the plug’s pins A and B. 
  2. Turn on the ignition but be careful not to rev up the engine. 
  3. Check the check engine light. It should flash once then twice, indicating a code 12 that represents the start of diagnostic mode. Each code will be iterated thrice. 
  4. Next, the check engine light will flash several times to indicate the trouble codes. You should note and record them with a piece of paper and pen. It would help if you got a partner to assist by writing down the codes or counting the number of flashes you might miss if you did the work independently.
  5. After the system displays all the error codes, it will flash a code 12 indicating the end of the diagnosis. 
  6. Switch off the ignition.
  7. Look up the error code in Google for clarity on the faulty component. Or you could come back to this page after this procedure and look it down below in our GM OBD1 code list. 
A process to use INNOVA 3123 OBD1 Code Reader to read GM code by Chavon Pitre

How to Clear GM OBD1 Codes

GM OBD1 codes are super easy to remove. You don’t have to do anything at times, and starting your car three or four times will eliminate the trouble codes. Nevertheless, there is a quicker method that is also relatively easy to execute. Here is how to do it:

  1. Locate the battery in your car, which should be located somewhere underneath the engine compartment. 
  2. Loosen the bolt reinforcing the plug to the negative terminal using a wrench. The negative terminal should have a minus symbol on it, and a black cable should be mounted using a plug on it. 
  3. Disconnect the cable from the negative terminal.
  4. Wait for 30 to 60 seconds before reconnecting the cable to the negative terminal. 
  5. Tighten the bolt with the wrench.
  6. Turn on the ignition and check if the error code is removed.
  7. If the check engine light is still flashing, repeat the above procedure a second time, only that this time you wait for 1 to 2 minutes.

Final Thoughts

In conclusion, our OBD1 codes list post is designed to be your ultimate guide in tackling diagnostic challenges for your classic GM vehicle.

We’d love to hear from you, so please feel free to leave a comment if you have any questions or wanna share your success stories about fixing your car’s problems. Your insights and experiences are incredibly valuable for any GM owner to address their issues.

And if you’re looking for the proper scanner for your vintage GM, take advantage of our enticing OBD1 scanner review.

Thanks for taking the time to read this post!


  1. GM OBD I Trouble Code Chart,
  2. List of all of the GM OBD1 codes,

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