Theory Of Operation

The PCM compares engine coolant temperature (ECT), intake air temperature (IAT), and ambient air temperature (AAT) under cold start conditions. Following a start to run delay time, the sensor values are compared. If the one sensor value is not within a specified range of the other two sensors, the value is determined to be irrational. Once the general temperature rationality is passed, the PCM determines that the general temperature sensor values are correct. The PCM compares the Oil Temperature Sensor value to a threshold based on the other temp sensor values. If the difference is greater than a calibrated value, the diagnostic fails.