Accessing and Reading Diagnostic Trouble Codes

ALDL connector:

The ECM incorporates a diagnostic program which, when activated, will flash codes stored in the trouble code memory through the ``Check Engine'' Lamp (CEL). A diagnostic connector, located under the instrument panel, is used to activate the self-diagnosis system. The connector, Fig. 6, allows access to system circuits and contains a test terminal which, when grounded with the ignition on and engine stopped, causes stored trouble codes to be flashed on the CEL.
When the test terminal is grounded (jumper terminal "A" to "B") with the ignition on and engine stopped, codes are displayed in numerical order, always starting with code 12 which indicates that the self-diagnosis system is operating properly. Trouble codes are displayed in the following manner: Code 12 is displayed by one flash, a short pause, then two flashes and a longer pause. Code 12 will be displayed 3 times in this manner, then any additional codes stored will be displayed 3 times each in a similar manner until all codes have been displayed. Once all codes have been displayed. The process will be repeated as long as the test terminal remains grounded.
After trouble codes have been noted, the ground wire should be removed from the test lead and the engine should be started. When the engine is started and there are no other codes except 12, the CEL should go out after approximately 5 seconds, indicating that the ECM has not detected a malfunction. However, if the ECM detects a current malfunction, the CEL will remain illuminated while the engine is running. A trouble code indicates a possible system malfunction. If a trouble code can be obtained, even if the CEL is off when the engine is running, the malfunction is intermittent and it may be necessary to road test the vehicle to observe conditions under which the malfunction occurs. Use of a SCAN tester greatly reduces the amount of time necessary to diagnose intermittent malfunctions.
Perform the ``Diagnostic Circuit Check'' in ``Testing Procedures'' to ensure the self-diagnosis system is operating properly. Follow steps as indicated in the Diagnostic Circuit Check and perform indicated corrections before proceeding with further tests.