Do you own a GM vehicle from 1980-1995? If yes, there is a high possibility you have an OBD1 connector (ALDL).
This connector is GM specific and applies to Pontiac, Chevrolet, GMC, Oldsmobile, Cadillac, and Buick, among others. The positive about the OBD1 connector is you can read the trouble codes at home without any diagnostic tools.
There are specific diagnostic tools for this type of connector, but they aren’t necessary – you can read them just as easily without a tool of any type.
If you have a newer car made after 1996, you have OBD2 codes in your car, and this article will not apply to your car. You may want to check out our other article which has OBD2 codes.
OBD1 Trouble Code List
This is a complete OBD1 trouble code list. The meanings for these codes can vary slightly depending on the car model you own but are generally the same for all manufacturers.
We will not take responsibility for errors in this code list. Refer to your repair manual for your car model’s trouble codes.
| Code | Description |
|---|---|
| 12 | System OK ( Diagnosis mode active ) |
| 13 | Oxygen O2 Sensor – Circuit open/no activity |
| 14 | Coolant Temperature sensor – Resistance too high 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 | Throttle position sensor – Out of range/Performance |
| 22 | Throttle position sensor – Low Voltage |
| 23 | Intake Air temp sensor – Out of range, low resistance |
| 24 | Vehicle speed sensor – Circuit fault |
| 25 | 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 | Manifold Air Pressure Sensor – Signal out of range, high resistance |
| 34 | 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 #2 – Solenoid error/faulty |
| 76 | Digital EGR #3 – Solenoid error/faulty |
| 77 | Digital EGR #1 – 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 |
Location of the OBD1 Connector
The OBD1 connector is typically found under the steering wheel and under the dashboard. It may be covered in black plastic that must be removed first. The connector is often black with 12 contact pins, making it easy to spot.
See the image above for an example of an OBD1 connector.
Wiring Diagram of the OBD1 Connector
The OBD1 connector has twelve pins, but not all are utilized for most vehicles. The standard pins (ground, power, & diagnostic) are the same for most vehicles.
Make certain your wires are hooked up correctly. If you are not careful, you may burn up the vehicle’s electrical circuits, control units, etc. This can get very expensive.
This is what the pinout of the OBD1 connector looks like for most vehicles. Please refer to your repair manual to find the exact vehicle 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 an OBD1 Code at Home
You can read an ODB1 code without a diagnostic tool by shorting the circuit of pin A and pin B. The vehicle will enter diagnostic mode and flash the “check engine Light” multiple times, and after counting the flashes, you’ll determine whether the vehicle has a diagnostic code.
How to read the OBD1 codes:
- Always connect a car battery charger to your car when you do any electrical work that requires the ignition is on. Low voltage can cause trouble with the troubleshooting process and may cause you to read multiple incorrect error codes.
- Place a jumper wire between PIN A and PIN B.
- Then let the jumper wire on PIN A and PIN B and turn your ignition ON, not start your engine.
- The check engine light will start to flash count your flashes. If you have multiple trouble codes, you will see a longer pause between the trouble codes vs a shorter pause to count the trouble codes. For example, code 16 = 1 flash * Pause * 6 flashes
- When the trouble codes have all been outputted, it will flash code 12 – 1 flash * Pause * 2 flashes
- Write down all the trouble code numbers you got and check the trouble code table further down in the article.
- Make sure to remove the jumper wire and battery charger.
OBD1 Code Reader
Numerous OBD1 scanners have made this job a little easier. When you use an OBD1 code scanner, you no longer need to short the connector and count the flashes on the vehicle’s dashboard.
You will get the problem in clear text on your scanner, which is a great money saver if you read lots of trouble codes on older vehicles.
There are many different code scanners on the market with a wide range of prices. Some of the scanners can read both OBD1 and OBD2 codes, but they are quite pricey.
An inexpensive scanner is the Innova 3123 that I found. The scanner gives you two options, Ford or GM vehicle.
How to reset OBD1 codes?
If you need to reset OBD1 codes on your vehicle, there are some simple steps to follow. First, disconnect the battery from the car.
The ECU will power every electronic gadget and electronic system in your car, including a factory alarm and theft deterrent radio system. Powering down the ECU will reset the radio, power seats, clock, and transmission shift points.
First, turn your car off. Next, the battery should be disconnected from the damaged terminal. Hold the battery disconnected for approximately one minute to erase the codes. Reinsert the battery to reboot the vehicle.
Once that has been done, you will find that the codes are erased, and the vehicle will run normally, if the issue persists try to take the battery off and reinstall it.
To reset the check engine light, there are two methods. The first method is to disconnect the battery and restart the vehicle. This is a simple way to clear codes on the vehicle’s ECU that should reset the check engine light. This method may not work on all vehicles.
Be careful not to disconnect the battery while disabling the OBD1 connector. If you do you may lose the radio presets. If you are going the DIY route, you can also read the codes with a pro-diagnostic tool.
Once that is completed, you will want to connect the OBD2 scanner to your vehicle. The OBD reader port is usually under the dashboard of the vehicle, and is commonly found somewhere near the steering wheel. After turning the vehicle off, you can connect the OBD reader to your vehicle and wait for a few seconds for it read codes.
The reader will then either display a single code, or list of codes. Once all the codes are erased, the vehicle will run normally and operate for driving.