OBD1 Codes – Trouble Code List & Reading (GM 2021)

Older car models before 1996 used the OBD1 diagnostic system. Here is how you can read these codes and what they mean

Obd1 Codes

Do you have a GM vehicle between 1980 to 1995? If so, there is a big chance your car has an OBD1 connector (ALDL).

This connector is unique and applies to Pontiac, Chevrolet, GMC, Oldsmobile, Cadillac, and Buick, and more. The positive thing with the OBD1 connector is you can read your trouble codes at home without any diagnostic tools.

There are special diagnostics tools for this kind of connector, but they are not necessary as you can read them just as well without a tool.

If you have a newer car after 1996, you have OBD2 codes in your vehicle and this article will not apply to your car. You can check our other article with OBD2 codes.

OBD1 Trouble Code List

Here is a complete OBD1 trouble code list. The OBD1 codes’ meanings can differ a bit depending on what car model you have, but most of them are the same among all car manufacturers. We do not take responsibility for any faults in this code list. Check your repair manual for the trouble codes of your car model.

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

OBD1 Connector Location

Obd1 Connector

The OBD1 connector is usually located under your steering wheel, under the dash. The connector may have a black plastic cover, which you have to remove before accessing it. The connector is often black and has 12 pins on it; pretty easy to identify.

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

OBD1 Connector Wirings Pinout

The OBD1 connector has 12 pins, but not all of them are used in most car models. The standard pins like the ground, power, and a diagnostic pin are the same in most car models though.

Make sure you connect your wires correctly, as if you are not careful, it may damage your vehicle’s electrical system and control units, which can be very expensive.

This is how the pinout of the OBD1 connector looks like in most car models, check your repair manual for the exact pinout:

Obd1 Pinout

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How to Read OBD1 Codes at Home

Obd1 Jump Wire

Our reading method without a diagnostic tool is to make short the circuit of pin A and pin B. The car will then enter the diagnostic mode and flash the “check engine Light” several times, and we should count the flashes to figure out the diagnostic codes.

How to read the OBD1 codes:

  1. Always connect a car battery charger to your car when doing any electrical work that requires the ignition to be on. Low voltage can cause a lot of trouble with the troubleshooting and many 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 trouble codes’ digits. 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 code 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 Reader

There are also a lot of OBD1 code readers on the market to make the job easier. With an OBD1 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 make 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. Some scanners 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:

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Written by:

Magnus is the owner and main author of MechanicBase. He has been working as a mechanic for over 10 years, and the majority of them specialized in advanced diagnostics and troubleshooting. Certified Automotive Diagnostic Technician.