Reading and Clearing Diagnostic Trouble Codes

Fig. 2 Trouble Code Test Lead Identification:

The ECM incorporates a diagnostic program which, when activated, will flash codes stored in the trouble code memory through the ``Check Engine'' lamp. A diagnostic connector, located under the instrument panel, is used to activate the self-diagnosis system. The connector, Fig. 2, allows access to system circuits and contains a diagnostic terminal which, when grounded with the ignition on and engine stopped, causes stored trouble codes to be flashed on the ``Check Engine'' lamp.

Fig. 3 Trouble Code Identification. Minimum Function System:

When the diagnostic terminal is grounded 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, Fig. 3, 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 ``Check Engine'' lamp is off when the engine is running, a ``System Performance Check'' should be performed and the indicated circuit should be inspected.
Use the appropriate diagnostic charts if trouble codes 13, 15, 24, 35, 41, 44, 45 and all 50 series codes can be obtained with the ``Check Engine'' lamp off. If any other trouble code is stored and the ``Check Engine'' light remains off, do not use the diagnostic charts for these codes, since the system is operating properly at the time. A System Performance Check and a thorough physical inspection of the circuit involved should be carried out if any of these codes are evident. Repair as necessary.