General Motors Trouble Codes

Introduction:

GM (Buick, Cadillac, Chevrolet, GMC, Pontiac, and Oldsmobile) OBD-I equipped vehicles are some of the easiest cars and trucks to pull codes from. You'll need nothing more than a paper clip as a diagnostic tool. This page is divided into general information that's relevant to all GM's of this era, and specific code listings for various platforms.

OBD-I vs OBD-II:

As was mentioned in the Trouble Code Project introduction, all vehicles made and sold after January 1, 1996, had to be compliant to the new OBD-II standard. Because this was incorporated as a running change, there are some GM vehicles built in 1994-1995 that have an OBD-II style connector, but still use the OBD-I system. To determine if a changeover-year vehicle is OBD-I or OBD-II, look under the hood on the emissions information stickers.

The ALDL:

OBD-I equipped GM's (with the exceptions listed above) have a connector known as the Assembly Line Data Link, which in addition to being used to retrieve trouble codes can also be connected to a scan tool or a laptop computer using a special interface cable. Some relay functions can also be triggered for testing using the ALDL. The connector is usually located under the dashboard and may have a plastic cap covering the terminals. It has two rows of female terminals with six per row. Here's how they're laid out:

F	E	D	C	B	A
G	H				M

A - Ground

B - Diagnostic Terminal (Engine)

C - A.I.R.

D - Check Engine Lamp

E - Serial Data

F - Torque Converter Lockup

G - Fuel Pump

H - Diagnostic Terminal (Anti-Lock Brakes)

M - Serial Data

The oddball vehicles with the OBD-II type connector (shown below) and OBD-I electronics can retrieve codes by connecting the #5 and #6 terminals.

OBD-II connector

Code Retrieval:

With the engine off, connect the A and B terminals (or the #5 and #6 terminals in the case of an OBD-I vehicle with the 16 pin OBD-II style connector) with your trusty paper clip. WITHOUT STARTING THE ENGINE, turn the key to the 'run' position and watch the "check engine" light. All GM codes are 2 digit, and each code will repeat three times before moving on to the next stored code. The format is "first digit, short pause, second digit, long pause", and the fact that each code is repeated three times makes it easy to figure out. The first and last code you'll get is "12", which indicates that the computer is in diagnostic mode. If you only get code 12, there are no faults stored in the system.

Resetting the ECU:

Once you've pulled codes and made any fixes, you can clear the codes and reset the ECU by removing the power to the computer for at least 30 seconds. You can do this by either disconnecting the positive battery terminal (but be sure you have the instructions and codes for resetting your radio's anti-theft code and your remote entry system, if so equipped) or by pulling the ECM fuse(s).

Once the computer has been reset, you'll need to drive the vehicle at part throttle under moderate acceleration, and allow it to come up to normal operating temperature to allow the system to relearn its base settings.

Code Definitions:

General Trouble Codes

Code	Definition
12	Normal Operation
13	Oxygen Sensor Circuit (open)
14	Coolant Sensor (Low Voltage)
15	Coolant Sensor (High Voltage)
16	System Voltage High
17	RPM Signal Error (3.8l C-body only)
21	Throttle Position Sensor (Low Voltage)
22	Throttle Position Sensor (High Voltage)
23	Manifold Air Temperature Sensor (High Voltage)
24	Vehicle Speed Sensor
25	Manifold Air Temperature Sensor (Low Voltage)
26	Quad Driver Error
27	Gear Switch Error (3.3l & 3.8l only)
28	Gear Switch Error (3.3l & 3.8l only)
29	Gear Switch Error (3.3l & 3.8l only)
31	1990 models - Turbo Purge Solenoid Error 1991 models - Park/Neutral Switch Error (3.3l & 3.8l only)
32	EGR Vacuum Control
33	MAP Sensor (Low Voltage)
34	MAP Sensor (High Voltage)
35	Idle Air Control Solenoid Error
36	Transmission Shift Circuit Error (PCM equipped models only)
38	Brake Switch Error (3.3l & 3.8l only)
39	Torque Converter Clutch Error
41	Cam Sensor Error (3.3l & 3.8l only)
42	Electronic Spark Timing (open or grounded)
43	Electronic Spark Control Error
44	Oxygen Sensor (Lean)
45	Oxygen Sensor (Rich)
46	Power Steering Pressure Switch or Anti Theft System Fault
47	PCM/BCM Lost Data (3.8l E-body only)
48	Misfire (3.3l & 3.8l only)
51	PROM Error
52	CALPAK Error or  Low Engine Oil Temperature (Corvette only)
53	System Over-Voltage
55	ECM / PCM Error
59	Anti-Theft System Error (3.8l only)
61	Degraded Oxygen Sensor (3.1l & 4.3l only)
62	Gear Switch Error (3.1l & 4.3l only)
63	EGR Flow Check Error (3.8l H-body only)
64	EGR Flow Check Error (3.8l H-body only)
65	EGR Flow Check Error (3.8l H-body only) or Fuel Injector Low Current (2.3l only)
66	AC Pressure Sensor Voltage Out of Range
67	AC Head Pressure Switch Error (3.8l only)

TBI (Throttle Body Injection) Trouble Codes

Code		Definition
12		Normal Operation
13		Oxygen Sensor Circuit (open)
14		Coolant Sensor (Low Voltage)
15		Coolant Sensor (High Voltage)
21		Throttle Position Sensor (Low Voltage)
22		Throttle Position Sensor (High Voltage)
24		Vehicle Speed Sensor
33		MAP Sensor (Low Voltage)
34		MAP Sensor (High Voltage)
42		Electronic Spark Timing
44		Oxygen Sensor (Lean)
45		Oxygen Sensor (Rich)
51		PROM Error
55		ECM Error
	Port Injection Trouble Codes

Code	Definition
12	Normal Operation
13	Oxygen Sensor Circuit (open)
14	Coolant Sensor (Low Voltage)
15	Coolant Sensor (High Voltage)
21	Throttle Position Sensor (Low Voltage)
22	Throttle Position Sensor (High Voltage)
24	Vehicle Speed Sensor
32	EGR Vacuum Control
33	MAP Sensor (Low Voltage)
34	MAP Sensor (High Voltage)
42	Electronic Spark Timing
43	Electronic Spark Control
44	Oxygen Sensor (Lean)
45	Oxygen Sensor (Rich)
51	PROM Error
52	CALPAK Error
55	ECM Error