Engine Coolant Temperature (ECT) Sensor Monitoring

Theory of Diagnostic Operation
There are two separate engine coolant temperature (ECT) diagnostics. In the first, the sensor is checked for an out-of-range condition where the sensor voltage is too low or too high indicating a short to ground, to the 5v reference voltage or an open circuit.

Sensors are also checked for activity to make sure that sensor output changes as the engine warms up. A sensor which had failed in such a way that the sensor voltage was a constant 2.3v would pass the out-of-range test but would fail this second test which expects to see a change in coolant temperature after the engine has been running for a while.

If the diagnostic fails on two consecutive driving cycles the MIL will be illuminated and a DTC set.

ECT Monitor Enabling Criteria
The ECT sensor Out-of-Range diagnostic is run continuously. The conditions which must be met in order for the test to run are:

^ Ignition on.
^ No DTC for intake air temperature.
^ Intake air temperature above -20°C.

The diagnostic to detect a lack of activity in the sensor circuit runs one time per trip and has another set of criteria. They include:

^ Engine is running.
^ Test has not already run.
^ Vehicle speed above 12 mph for more than 400 seconds.
^ Coolant temperature at the beginning of the trip less than 40°C.
^ Coolant temperature has not risen more than 20°C since the start.