Engine Runs/Will Not Perform Self-Diagnostics

ENGINE RUNS/WILL NOT PERFORM DIAGNOSTIC TESTS
The way in which the engine runs can be a clue to the cause of a test problem. If the engine seems to run normally but will not complete a test or display computer data, the cause may be a wiring problem to the diagnostic connector that does not affect the rest of the system. If the engine runs poorly, the cause may be a poor control system ground or a computer voltage problem that affects the logic module or the entire system.

Refer to, "Testing The Diagnostic Connector", for instructions on checking the connector and its circuits. Testing the Diagnostic Connector