Introduction to On Board Diagnostics (II)

Functionality of PowertrainControl Module (PCM) inOBD IIOxygen sensor: The PCM shall monitor the output voltage, the response rate, andother parameters that can affect emissions, and all fuel control oxygen sensors formalfunction. The algorithm involves monitoring for short circuit, or breaks, andmonitoring the switching frequency of the closed-loop control. If this is too slowor too fast relative to the limit cycle frequency of the air/fuel mixture, then theoxygen sensor is deemed defective. The PCM illuminates the MIL in the event of afault and stores the DTC and diagnostic data in memory. Heated sensors aremonitored using heater current, voltage, and sensor temperature .Evaporative system: The PCM shall control the air flow of the complete evaporativesystem. The PCM shall also monitor the emission of HC vapors into the atmosphereby performing a pressure check and a vacuum check of the purge valve,and the canister valve, using intrusive purge operations. The algorithm is two fold.At idle position, the purge valve is activated and the lambda sensor is monitoredfor its reaction which should indicate a rich reading (high voltage of 900 mv). For leakdetection of the evaporative system, the canister valve is closed, and the canister pressure isdecreased to about about -1.5 KPa. Then the complete system is turned off and the pressurewithin the canister is monitored for variation with time. The pressure gradient, together withother parameters like the amount of fuel, may indicate a leak. If the leak persists for twoconsecutive cycles, the MIL is illuminated.