Theory of Operation

The Engine Coolant Temperature Sensor (ECT) is located near the thermostat housing and is used to measure the engine coolant temperature. The Engine Coolant Temperature Sensor receives 5-Volts and a sensor ground from the Powertrain Control Module (PCM). The PCM monitors the change in voltage to determine the coolant temperature. There are two parts to this fault code, a key on check and a rationality check. After an eight hour cold soak, at key on the readings for the EGR Temperature, Inlet Air Temperature and Engine Coolant Temperature Sensors are all compared. If the temperatures differ more than a calibrated amount, then the appropriate sensor fault code would be recorded. This monitor looks for all the sensors to be grouped on one temperature or, in the case that the monitor fails, two sensors grouped at one temperature and one outlier. In the case that all three sensor values are distributed over a range of temperatures this diagnostic will not run. A block heater is one possible cause of such a distribution. The Coolant Temperature Sensor rationality check looks at the temperature reading from the sensor over time and ensures that it changes with engine running. If the sensor reading does not change over a calibrated time limit, the fault will be recorded. Both the key-on and rationality portions of this monitor require that the diagnostic fails in two consecutive drive cycles before the MIL lamp is lit. During this time the PCM uses a default value for the Coolant Temperature Sensor. The PCM turns off the MIL lamp when the diagnostic runs and passes in four consecutive drive cycles.