Accessing and Reading Diagnostic Trouble Codes

The provision for communicating with the ECM is the Assembly Line Diagnostic Link (ALDL) connector. It is usually located under the instrument panel and is sometimes covered by a plastic cover labelled "DIAGNOSTIC CONNECTOR." It is used in the assembly plant to receive information in checking that the engine is operating properly before it leaves the plant. The code(s) stored in the ECM's memory can be read either through a hand-held diagnostic scanner plugged into the ALDL connector or by counting the number of flashes of the "SERVICE ENGINE SOON" light when the diagnostic terminal of the ALDL connector is grounded. The ALDL connector terminal "B" (diagnostic terminal) is the second terminal from the right of the ALDL connector's top row. The terminal is most easily grounded by connecting it to terminal "A" (internal ECM ground), the terminal to the right of terminal "B" on the top row of the ALDL connector.

Once terminals "A" and "B" have been connected the ignition switch must be moved to the "ON" position, with the engine NOT running. At this point, the "SERVICE ENGINE SOON" light should flash code 12 three times consecutively. This would be the following flash sequence: "flash, pause, flash-flash, long pause, flash, pause, flash-flash, long pause, flash, pause, flash-flash." Code 12 indicates that the ECM's diagnostic system is operating. If code 12 is not indicated, a problem is present within the diagnostic system itself, and should be addressed by consulting the appropriate diagnostic chart.

Following the output of Code 12, the "Service Engine Soon" light will indicate a diagnostic trouble code three times if a code is present, or it will simply continue to output Code 12. If more than one diagnostic code has been stored in the ECM's memory, the codes will be output from the lowest to the highest, with each code being displayed three times.