Description of On-Board Diagnostics

OPERATION
The Engine Control Module monitors the input/output signals (some signals at all times and the others under specified conditions) of the Engine Control Module. When it is noticed that an irregularity has continued for a specified time or longer from when the irregular signal is initially monitored, passing a predetermined value, the Engine Control Module judges that an irregularity has occurred, memorizes the diagnostic trouble code, and outputs the signal to the on-board diagnostic output terminals. Certain irregularities will also illuminate the Check Engine/Malfunction Indicator light. There are multiple on-board diagnostic items, including the normal state, and the diagnostic results can be read out with a voltmeter or scan tool. Moreover, since memorization of the diagnostic trouble codes is backed up directly by the battery, the diagnostic results are memorized even if the ignition key is turned off. The diagnostic trouble codes will, however, be erased when the battery terminal or the Engine Control Module connector is disconnected. The diagnostic trouble codes are also erased by setting the ignition switch to the ON position and then sending the diagnostic-trouble-code erase signal from the scan tool to the engine control module.

The testing procedures contained in this area will identify any codes that are present, describe what they mean, test for causes and give you a way to clear those codes after the problem has been repaired. If no code is present, there are tests available in the Diagnostic Charts section for those problems that DO NOT set a trouble code.

NOTE: If the sensor connector is disconnected while the ignition switch is ON, a diagnostic trouble code is recorded. In this instance, either send the diagnostic-trouble-code erase signal from the scan tool to the Engine Control Module, or disconnect the negative battery terminal for ten seconds or longer to erase the diagnostic memory.

CAUTION: An ignition timing adjustment signal diagnostic trouble code is output when the ignition timing adjustment terminal line is short-circuited to the ground. Therefore, the diagnostic trouble code is output when the ignition timing adjustment terminal is grounded. However, this is not a malfunction.