On-Board Diagnostics

The Engine Control Module (ECM) monitors several different engine control system circuits. If malfunction occurs which affects vehicle emissions, ECM will store a Diagnostic Trouble Code (DTC) when malfunction is detected after engine is restarted on all failures except excessive time to enter closed loop. On excessive time to enter closed loop, ECM will store a DTC when malfunction is first detected. When DTC is stored, ECM will use a substitute value for the failed component in its calculations to continue engine operation. In this condition, commonly know as limp-in mode, the vehicle runs but driveability will not be optimum.

The ECM also records and stores engine operating conditions when the malfunction occurred. This information is referred to as freeze frame data. Freeze frame data may be obtained using scan tool, and used for simulating tests for trouble shooting. Freeze frame data is available for:

• Calculating Load Value
• DTC Data Recording
• Engine Coolant Temperature
• Engine Speed
• Fuel Control Condition Such As Open Or Closed Loop
• Long Term Fuel Compensation (Fuel Trim)
• Short Term Fuel Compensation (Fuel Trim)
• Vehicle Speed

DTCs may be retrieved for system diagnosis. System malfunctions are identified as either hard failures or intermittent failures.