On-Board Diagnostics Overview

ON-BOARD DIAGNOSTICS (OBD)
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 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. For non-emission related faults, the memory is cleared each time the engine is started, so if a fault is suspected, codes should be displayed and recorded after the fault has been observed and before the engine is re-started.