All vehicles covered by this information have self-diagnostic capabilities. Malfunctions in the engine control system are found through the Self-Test procedure. The vehicles covered by this information use the Electronic Engine Control-IV (EEC-IV) system, which utilizes a Self-Test divided into 3 specialized tests: Key On Engine Off Self-Test, Engine Running Self-Test and Continuous Self-Test. The Self-Test is not a conclusive test by itself, but is used as a part of a functional Quick Test diagnostic procedure. The computer control module stores the Self-Test program in its permanent memory. When activated, it checks the EEC-IV system by testing its memory integrity and processing capability, then verifies that various sensors and actuators are connected and operating properly. The Key On Engine Off and Engine Running Self-Tests are functional tests which only detect faults present at the time of the Self-Test. Continuous Self-Test is an ongoing test that stores fault information in Keep Alive Memory (KAM) for retrieval at a later time.
Fault information is communicated through the Self-Test service codes. These service codes are 2-digit or 3-digit numbers representing the results of the Self-Test. The service codes are transmitted on the Self-Test output line found in the vehicle Self-Test connector. They are in the form of timed pulses and can be read on a voltmeter, STAR or SUPER STAR II tester and the malfunction indicator light.