OBD1 Codes – Reading & Code List

flashing check engine lightDo you have a GM car from 1980 – 1995?

If so, there is a big chance that you have the OBD1 connector (ALDL).

This connector is unique and applies to Pontiac, Chevrolet, GMC, Oldsmobile, Cadillac, and Buick. The good thing if you have this connector, is that you can read you trouble codes at home without any diagnostic tools.

There are special diagnostics tools for this kind of connector, but it is not necessary as you will read them as good without a tool.

If you have a newer car and need an OBD2 diagnostic tool you can check our other articles. OBD2 codes list.

OBD1 Connector location

obd1 connector

The connector is usually located under your steering wheel, under the dash. The connector may have a black plastic cover over it and you may have to remove some covers to reach it. The connector is often black and has 12 pins on it, pretty easy to identify.

Check the image above to see how an OBD1 connector looks like

OBD1 Connector wirings pinout

The OBD1 connector has 12 pins, but not all are used on the most cars. The standard pins like the ground, power, a diagnostic pin is the same between a lot of different car manufacturers. Make sure you connect your wires right, as if you are not careful, it may result in damages to your vehicles electrical system and control units.

Pinout of the OBD1 connector

OBD1 pinout

How to read OBD1 codes at home

OBD1 jump wireOur method to read the codes without a diagnostic tool is to make a “short” between pin A and pin B. The car will then enter the diagnostic mode and will flash the “Check engine Light” several times and we should count the flashes to see the diagnostic codes.

How to read the codes:

  1. Always connect a car battery charger to your car when doing any electrical works that require the ignition on. Low voltage can cause a lot of trouble with the troubleshooting and a lot of error codes that we do not want.
  2. Put a jumper wire between PIN A and PIN B.
  3. Let the jumper wire sit there and turn the ignition to ON. Do not start the engine
  4. The Check engine light will begin to flash. Count the flashes. There will be a longer pause between the codes if you have several trouble codes and a shorter pause between the digits of the trouble codes. For example code 16 = 1 flash * Pause * 6 flashes.
  5. When all trouble codes are outputted, it will flash the code 12 – 1 flash * Pause * 2 flashes.
  6. Write down all the trouble codes numbers you got and check the trouble code table further down in the article.
  7. Remove the jumper wire and car battery charger.


OBD1 Code Reader

obd1 code readerThere are also a lot of OBD1 code readers on the market to make the job simpler. With a code scanner, you do not have to short the connector and you do not have to count the flashes on the dashboard. You will get the problem in clear text on your device and this is, of course, a great investment to do if you are reading a lot of trouble codes from older vehicles. 

There are several different code scanners on the market in all different price ranges. There are scanners that can read both OBD1 and OBD2 codes but these are pretty expensive. One really affordable scanner that I found is the Innova 3123. You have two different options: Ford or GM Vehicles you can choose from. If you want to take a further look at the scanner you can find it here on Amazon:  INNOVA 3123 GM OBD1 Code ReaderOBD1 Codes - Reading & Code List

OBD1 codes list

Here is a list of what the OBD1 codes mean. The meanings of the OBD1 codes can differ a bit depending on what car model you have, but the most of them are the same among all car manufacturers.

  • 12. System OK ( Diagnosis mode active )
  • 13. Oxygen O2 Sensor – Circuit open/no activity
  • 14. Coolant Temperature sensor – Too High resistance or shorted circuit
  • 15. Coolant Temperature Sensor – Circuit low or Open
  • 16. Direct ignition system (DIS) – faulty circuit / shorted circuit
  • 17. Camshaft Position – Sensor/Circuit/Timing error
  • 18. Camshaft or Crankshaft – Sensor/Circuit/Timing error
  • 19. Crankshaft sensor – Sensor/Circuit/Timing error
  • 21. TPS ( Throttle position sensor ) – Out of range/Performance
  • 22. TPS ( Throttle position sensor ) – Low Voltage
  • 23. IATS ( Intake Air temp sensor ) – Out of range, low resistance
  • 24. VSS ( Vehicle speed sensor ) – Circuit fault
  • 25. IATS ( Intake Air temp sensor ) – Out of range, high resistance
  • 26. QDM A ( Quad-driver module ) – Circuit #1 fault
  • 27. QDM A ( Quad-driver module ) – 2nd gear circuit
  • 28. QDM A ( Quad-driver module ) – Circuit #2 fault
  • 29. QDM A ( Quad driver module ) – 4th gear circuit
  • 31. Wastegate solenoid – Circuit fault
  • 32. EGR ( Exhaust Gas Recirculation ) – Circuit fault
  • 33. Map ( Manifold Air Pressure Sensor ) – Signal out of range, high resistance
  • 34. Map ( Manifold Air Pressure Sensor ) – Signal out of range, low resistance
  • 35. Idle air control valve / Sensor – circuit fault
  • 36. Ignition system – Circuit error/fault
  • 38. Brake input ( Brake switch sensor ) – circuit fault
  • 39. Clutch input ( Clutch sensor ) – circuit fault
  • 41. Camshaft sensor – Circuit fault or ignition control – circuit fault/error  ( Depending on the car model )
  • 42. EST (Electronic spark timing) – circuit grounded/shorted
  • 43. Knock sensor / Electronic spark control – circuit fault error  ( Depending on the car model )
  • 44. Oxygen O2 sensor – Lean Mixture
  • 45. Oxygen O 2sensor – Rich Mixture
  • 46. Pass-key II – circuit or Power steering pressure switch – circuit fault error  ( Depending on the car model )
  • 47. PCM data – circuit error
  • 48. Misfire ( diagnosis )
  • 51. Calibration error – mem-cal, ecm or eprom failure
  • 52. Engine oil temperature circuit – low temperature indicated / Circuit error
  • 53. Battery voltage – low/high/error or Egr valve solenoid 1 – Circuit error  ( Depending on the car model )
  • 54. Fuel Pump – Circuit error or Egr valve solenoid 2 – Circuit error ( Depending on the car model )
  • 55. ECM PCM – Circuit error or EGR valve solenoid 3 – Circuit error ( Depending on the car model )
  • 56. Quad-driver module (QDM B) – circuit error
  • 57. Boost control – Error / Faulty
  • 58. Vehicle anti-theft system ( VATS ) – circuit faulty
  • 61. AC (Air conditioning) system performance or degraded oxygen sensor signal ( Depending on the car model )
  • 62. Engine oil temperature sensor – high temperature indicated
  • 63. Oxygen O2 sensor right side – circuit open or MAP ( Manifold air pressure sensor ) – out of range ( Depending on the car model )
  • 64. Oxygen O2  sensor right side – lean mixture indicated
  • 65. Oxygen O2 sensor right side – rich mixture indicated
  • 66. A/C (Air conditioning ) pressure sensor – circuit low pressure
  • 67. A/C (Air conditioning ) pressure sensor – circuit or a/c clutch – circuit failure ( Depending on the car model )
  • 68. A/C (Air conditioning ) compressor relay – circuit failure error
  • 69. A/C (Air conditioning ) clutch – circuit / pressure high
  • 70. A/C (Air conditioning ) refrigerant pressure – circuit high
  • 71. A/C (Air conditioning ) evaporator temperature sensor – circuit low
  • 72. Gear selector switch – Circuit error / Fault
  • 73. A/C (Air conditioning) Evaporator temperature – circuit high
  • 75. Digital EGR #1 – solenoid error/faulty
  • 76. Digital EGR #2 – solenoid error/faulty
  • 77. Digital EGR #3 – solenoid error/faulty
  • 79. Vehicle speed sensor (SS) – circuit signal high resistance
  • 80. Vehicle speed sensor (vss) – circuit signal low resistance
  • 81. Brake input data – circuit faulty
  • 82. Ignition control (IC) 3X – signal error/faulty circuit
  • 85. Prom –  error/faulty circuit
  • 86. Analog/digital – pcm error /faulty circuit
  • 87. Eprom – error / Faulty circuit
  • 99. Power management – Error / Faulty circuit


OBD1 codes can be a bit confusing and not as good precision as OBD2 codes. But you will get a hint of where to start your troubleshooting.

Always connect a car battery charger when you are working with your vehicle.

If you have any questions that did not get answered in this article, comment down below. If you have other car questions check our other articles or ask us a question at our home page.

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14 thoughts on “OBD1 Codes – Reading & Code List”

  1. Obd1 trouble codes
    1991 jeep wrangler yj 4.0l
    22: Coolant sensor signal out of range
    24: throttle sensor signal out of range

    What is the definition or meaning of sensor signal out of range? And what would be the best way to fix the problem and clear the codes?
    -any and all info will help! Please. And thank you.

  2. @Brett Ritter

    The engine control unit has a range of the signal from example 1-5 volts at the certain temperature / speed. If the sensor is sending a signal out of that range, a trouble code would be set. This could be caused by a faulty sensor, faulty wirings or a faulty ECM (Rare). It can also occur due to low voltage and bad ground. I would start with measuring the temperature sensor and make sure it has the right resistance for the temperature. You can normally find a temperature chart in the repair manual. You can also remove the trouble codes and see if they’re coming back!

  3. Hi. I am pleased to find your trouble code site. Thank you. My name is also Joshua. I own a 90′ GMC. 5.7L. Can you confirm that the codes are shown through the “RED -BRAKE LIGHT” flashing?

    • The brake light flashing in a pattern may have other trouble codes. I recommend you to find a list of the ABS codes or use an OBD1 code scanner instead to be 100% sure 🙂

  4. hi josh
    i have an 84 oldsmobile (family car since 1984) and have used the diagnostic link regularly
    most recently the dashboard doesn’t flash anymore
    it still makes that pulsing buzzy noise but no blinking lights
    car runs great otherwise
    any reasons why this is happening?

    • If I can understand your question correctly, there may be a broken bulb inside the instrument cluster. There are several videos on the web of how to replace them or check them with a multimeter if you would like to 🙂

  5. Hello, thanks for making this web page, it really helps. I have identified my error code as a code 36, “ignition system circuit error”. So what exactly does that mean? Missing 12v on a specific input pin on a electric device? Where do I start looking?

    My vehicle is a 1995 Winnebago Itasca Sunrise, 7.4L TBI engine. This is my home and I’ve had this error since I got the unit a year ago. Seems to run ok, maybe a little rich. Any help would be greatly appreciated. I’m a pretty good mechanic, just not a lot of experience on a chevy 7.4L.

    Thank You so much for your time..

    • Hi, I’m not familiar at all with this car engine. However, this trouble codes does often means that there is a wiring problem to the distributor or the ignition coil. It could also be a caused distributor/coil. To measure this you need a wiring diagram to find out which pins should get 12+ b+/12v ign and ground/Signal

  6. 86 Corvette
    Engine code 8
    Seems as if it is in limp mode
    I jumped A and B terminals
    Did not not flash any thing.
    Also tells me to upshift in park lol.
    Check engine light randomly flashing while idling…
    All of it is confusing, I think I have a pcm issue.
    Thanks Dennis Meyers

    • It may be a problem with the transmission control unit or a speed sensor/shift lever. To me it sounds like that the Transmission control unit does not get any speed information and therefore thinks it speeding while staying in park.

  7. 94 caprice 4.3L V8. OBD1 trouble codes can not be extracted. I jumper A and B top right ports and turn key on then the Fans turn on to high and no check engine light flashing.
    Also this car is known for when giving hard acceleration it kicks on check engine light along with both cooling fans on high speed. After this happens you can pull over and turn car off, start it back up and all is fine until the next hard acceleration.

    Would anyone have an idea as to what’s going on?


    • If the cooling fans are starting suddenly, there might be a problem with a coolant sensor. Did you tried with an OBD1 scanner like the INNOVA 3123? Check all power wires and ground to the engine control unit.

  8. Hello,I’m Oyin and my sweet baby Nissan primera 2.0 is giving me a heartache by blinking the check engine light at its will and resetting itself after removing battery terminals for almost 24hours …so disappointing at times. Kindly help me with suggested solutions… I’ve also been advice to get rid of the code but so worried that this decision might affect its brain box and maybe other functions.. Thanks in anticipation

  9. I have a Buick 1994 limited edition roadmaster. When I wire jump it off the service light won’t stop blinking it blinked 37 times back to back then didn’t blink anymore
    What should I do?

  10. Due to a very high demand and high ammount of comments, you have to wait for some time for your car questions to get answered. If you want to get fast answers from a certified master technician you can ask your questions here:
    Ask A Mechanic

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