GM OBD-I Codes

How to read GM codes

1. Find the ALDL connector under the dash, driver side.
2. Jump terminals A and B with a paperclip. Key off first.
3. Turn the key on, engine off. Count the check-engine flashes.
4. Code 12 (one flash, pause, two flashes) means the system is ready. Fault codes follow, each flashed 3 times.
5. Look up each code below. Remove the jumper before starting the engine.

Popular GM codes

• 13: Oxygen Sensor Circuit Open Or No Activity
• 14: Engine Coolant Temp Sensor Error High Temp Indicated
• 21: Throttle Position Sensor Error Signal High
• 24: Vehicle Speed Sensor Error
• 33: MAP Sensor Circuit Error Signal High
• 43: Knock Sensor Error
• 44: Oxygen Sensor Error Lean Condition Indicated
• 51: PROM Error

GM (68)

Powertrain - Computer and Auxiliary Controls

• 12
• 16
• 26
• 27
• 29
• 46
• 47
• 51
• 52
• 53
• 55
• 56
• 61
• 66
• 67
• 68
• 69
• 70
• 71
• 73
• 85
• 86
• 87
• 98
• 99

Powertrain - Fuel/Air Metering

• 13
• 14
• 15
• 21
• 22
• 23
• 25
• 33
• 34
• 35
• 36
• 44
• 45
• 54
• 57
• 63
• 64
• 65

Powertrain - Ignition

• 17
• 19
• 41
• 42
• 43
• 48
• 82

Powertrain - Transmission

• 24
• 28
• 31
• 37
• 38
• 39
• 58
• 59
• 72
• 79
• 80
• 90
• 93
• 96

Powertrain - Auxiliary Emissions Controls

• 32
• 75
• 76
• 77