Models that employ both measurable engine parameters as well as predictable engine parameters may be used to determine when a thermostat is malfunctioning before the thermostat malfunction results in an engine breakdown. Particular models may be used to provide an estimated coolant temperature and an estimated thermostat position. The estimated coolant temperature can be compared to an actual measured engine coolant temperature. The estimated thermostat position can be evaluated with respect to what the thermostat position should be given a particular engine coolant temperature. In some cases, comparison between a healthy model and a faulty model may be used to ascertain thermostat health.