Thermostat Monitoring Description
The diagnostic checks for a partially open thermostat, under conditions when the thermostat would be expected to be shut. See Fig 1 .
A second coolant temperature sensor is installed in the outlet from the radiator. If the enablement criteria are met and the engine coolant temperature is less than the normal thermostat opening temperature, then the diagnostic will run.
The diagnostic looks at the difference between engine coolant temperature and the radiator outlet temperature. This gives the temperature drop across the radiator.
If the temperature drop is less than a threshold, and there is flow across the radiator, this points to leakage through the thermostat.