On-Board Diagnostics Overview

ON-BOARD DIAGNOSTICS II (OBD II)
The ECM monitors sensor inputs and its own output signals, comparing these values with those stored in its memory. If a signal deviates greatly from what the ECM "knows" is the correct value, a diagnostic trouble code is stored in the fault memory. The OBD II system uses a standardized set of trouble codes and a standard 16-pin Data Link Connector. The ECM may substitute a good signal value from its memory to provide good driveability. Because of this substitution of signals, the operator may not be aware that there is a problem. If the fault is emission related, the ECM will store the fault code until the memory is erased and will illuminate the "Check Engine" light in the instrument panel. Check engine light will remain lite anytime the vehicle is in operation until the fault is corrected and the fault memory is cleared. In the case of a fault that may cause additional vehicle damage the "Check Engine" light will flash on and off until the fault is no longer detected or the fault is corrected and the trouble code memory erased.