GM OBD I Diagnostic Codes

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What is a DTC?

A DTC is a diagnostic Trouble Code. The computers used on Third Generation F-bodies are considered OBD-I (First Generation On Board Diagnostics).They are called that, because not only do they control the engine management system, but also is capable of performing a self-diagnostics of the engine management For example, the ECM (Electronic Control Module) uses the Oxygen Sensor (O2 Sensor) to monitor exhaust gas for being to rich or lean. If the computer feels that the conditions are right for the sensor to be malfunctioning, it will set a DTC 13 (O2 Sensor circuit fault). Once a DTC is set, the Check engine light will illuminate. There are many acronyms for the Check engine light such as CEL, MIL, SES, but we will use CEL (check engine light).

How do I pull DTC's?

This is simple. You don't even need an expensive scan-tool or code reader, just a simple paper clip. Locate the ALDL (Assembly Line Diagnostic Lead). It is usually under the drivers side of the dash. It may have a cover over it, and may even say "Diagnostic Connector".

This is the ALDL.  The two terminals that you need to be concerned with is in the picture.  It is terminals "A" and "B".  Simply turn the Ignition to "Run" (Do not start the vehicle) and put the paper clip in the two terminals.  Now watch your Check Engine Light.  It will start flashing.  It will look like Morse Code, but it isn't, and is real easy to interpret.

The first code that will flash will be a code 12.  It will be  "flash, pause, flash, flash, pause, pause," and repeat the cycle three times.  The ECM will flash each code three times, and move on to the next.  If you have no codes stored, it will keep flashing code 12.  Code 12 simply tells you the ECM is not receiving any pulses from the distributor (since the car is not running).  If you pull codes with a scantool and the car is running, this code will not show up.  Its no big deal, it lets you know the ECM is in the "Field Service Mode"  You will also notice the cooling fans come on, this is normal.

Tip: If you insert the jumper after you have started the car's engine it will read rich/lean by the CEL. When the light is on longer than off it means rich, off longer than on means lean.

What do I do with the Codes?

This is less simple.  OBD-I codes are very vague, and diagnosis sometimes isn't that simple.  Most people think that a code 32 (EGR system) means the EGR valve is bad and needs replaced.  This isn't always so!!!  There are many components in the EGR system that could cause the code to set OTHER than the EGR valve itself!  Below is a list of codes and what they mean. Follow the code number for more in depth information about each code.

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Code:           Meaning:

12	- No Distributor pulses
13	- Oxygen sensor
14	- Coolant Temp sensor circuit
15	- Coolant Temp sensor circuit
21	- Throttle Position Sensor
22	- Throttle Position Sensor
23	- Manifold Air temperature sensor
24	- VSS (Vehicle Speed sensor)
25	- Manifold Air temperature sensor
26	- Quad Driver Fault
31	- Wastegate Solenoid Circuit Error
32	- EGR system
33	- MAP sensor or MAF sensor depending on engine
34	- MAP sensor or MAF sensor depending on engine
35	- Idle Air Control Valve
36	- MAF Burn-off circuit
41	- No Distributor pulses, cylinder select error
42	- Electronic Spark Timing or Bypass fault
43	- Electronic Spark Control fault
44	- Lean exhaust
45	- Rich exhaust
46	- VATS system fault
51	- PROM fault
52	- CALPAK fault
53	- System over-voltage
54	- Fuel pump circuit
55	- ECM fault
61	- Oxygen sensor fault (degraded sensor)
66	- Air Conditioning Pressure Switch Fault

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