On-Board Diagnostics

Signals from various sensors are monitored continuously. If certain signals are lost or become faulty, PCM will adopt fixed substitute values (limp-home mode) to enable vehicle to be driven for certain failures. Substitute values can be adopted for Engine Coolant Temperature (ECT) sensor signal, Throttle Position (TP) sensor signal, Mass Airflow (MAF) sensor signal, and oxygen sensor (HO2S) signal.

NOTE: To avoid confusion if there is an intermittent malfunction with Malfunction Indicator Light (MIL), carefully read following text.

PCM stores Diagnostic Trouble Codes (DTCs) differently in accordance with a predetermined pattern. An emission related fault must be present for a predetermined number of intervals before PCM permanently stores a DTC and illuminates MIL. If fault is cleared before a specified interval has passed, DTC is erased from memory. There are 3 main types of intervals:

Trip

All diagnostic functions have been performed.

Running Cycle

Fuel trim (closed loop) has been initiated.

Time

Time in seconds.

Faults are recorded in PCM memory in the form of DTCs. Codes can be displayed using a generic scan tester connected to On-Board Diagnostics II (OBD-II) Data Link Connector (DLC).