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OBD1 codes – Reading & Code List

In OBD2 Trouble Codes by Josh S2 Comments

flashing check engine lightDo you have a GM car between 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

  • A = Ground
  • B = Diagnosis Terminal
  • C = A.I.R
  • D = Service engine soon light
  • E = Serial Data
  • F = T.C.C
  • G = Fuel pump
  • H = Brake Speed input
  • J = None
  • K = None
  • L = None
  • M = Serial Data


How to read OBD1 codes at home

OBD1 jump wire

Our 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 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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  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. Author

    @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!

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