Diagnosis of Thermostat
P0128
Due to a missing or stuck-open thermostat the rise of the engine coolant temperature is less then normally expected. This effect will be used by the diagnosis to detect a malfunction of the thermostat as follows.
A reference engine coolant temperature (tmotref, engine coolant temperature at maximum heat radiation with a closed thermostat) is calculated with the help of a temperature model. If the monitoring conditions are enabled, the difference between this calculated reference temperature and the actual measured engine coolant temperature (tmotref - tmot) is determined.
If the difference exceeds a calibrated threshold for a calibrated period of time and the coolant temperature sensor is diagnosed without any error, a malfunction is detected and a fault code is set. If the difference remains below the calibrated threshold for the above mentioned calibrated period of time, an error from a previous warm up cycle is cleared as soon as a calibrated coolant temperature is reached.