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November 18, 2018 
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Engine Trouble Codes (OBD-I Cars)
Last Updated: March 18, 2006

Below is a table of GM engine codes for as listed in my 1991 factory service manual. This should apply to all OBD-I vehicles.

CODEMEANING
12Normal code with ignition on and engine off. Indicates no distributor reference pulse to the ECM. Not stored in memory.
13Oxygen Sensor Circuit (Open Circuit)
14Coolant Temperature Sensor Circuit (High Temp. Indicated)
15Coolant Temperature Sensor Circuit (Low Temp. Indicated)
21Throttle Position Sensor (TPS) Circuit (Signal Voltage High)
22Throttle Position Sensor (TPS) Circuit (Signal Voltage Low)
23Intake Air Temperature (IAT) Sensor Circuit (Low Temp. Indicated)
24Vehicle Speed Sensor (VSS) Circuit
25Intake Air Temperature (IAT) Sensor Circuit (High Temp. Indicated)
32Digital EGR / Exhaust Gas Recirculation (EGR) System
33Manifold Absolute Pressure (MAP) Sensor Circuit (Signal Voltage High - Low Vacuum)
34Manifold Absolute Pressure (MAP) SensorCircuit (Signal Voltage Low - High Vacuum)
35Idle Air Control (IAC) Circuit
41Cylinder Select Error (Faulty or Incorrect MEMCAL)
42Electronic Spark Timing (EST) Circuit
43Electronic Spark Control (ESC) Circuit
44Oxygen Sensor Circuit (Lean Exhaust Indicated)
45Oxygen Sensor Circuit (Rich Exhaust Indicated)
51MEMCAL Error (Faulty or Incorrect MEMCAL)
52Cal-Pak Error (Faulty or Incorrect Cal-Pak)
53System Over Voltage
54Fuel Pump Circuit (Low Voltage)
61Degraded Oxygen Sensor
62Transaxle Gear Switch Circuit
66A/C Pressure Sensor Circuit


contributed by: Dustin Bainbridge
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