Retrieving Trouble Codes
When the vehicle is first started, the check engine light should turn on, but will turn off after a few moments. If it comes on again while the engine is running, then the ECM has noticed an unusual condition or problem. In response to that, it will turn on the check engine light and store a code. These codes are in the memory of the computer, and can be retrieved later. To find out what trouble codes have been set, follow the following procedure:
Turn the ignition on, but DO NOT start the car. Locate the ALDL connector. This is mounted above the driver's feet on vehicles factory equipped with TPI. Using a small segment of wire, jumper the A and B terminals. These would be the two terminals on the top right. Now observe the check engine light. The light will first flash once, then pause briefly. Next it will flash twice, then pause for a long time. This means it is reporting a code 12. It will repeat each code three times, then proceed to the next code in memory. If there are no more, it will start all over again from the beginning. Once you are finished retrieving codes, remove the jumper. NEVER start the car with the jumper in place, as you may damage the ECM. You may use the trouble code chart listed under Tech Articles to find out what each code means.
Note: It is normal for the ECM to report a code 12. Code 12 just means that the distributor is not firing (its not supposed to because the car is off). If there are no problems, the ECM will just continue to show a code 12 repeatedly.
Clearing Trouble Codes
If you have replaced any components or performed any work on the car, it might be convenient to clear any previous trouble codes. To do so, make sure the ignition is off, and disconnect the negative battery cable. Leave it off for 30 seconds to a minute, then reconnect it. Any stored codes will no longer be stored in the computer's memory.