Theory Of Operation

This diagnostic detects Thermostat stuck open failures. This can result in coolant warm-up delay or coolant temperature drop during engine operation. The warm-up behavior test monitors the measured engine coolant temperature for warm-up after engine start. The measured temperature is compared to a model of engine coolant temperature based on normal warm up characteristics. The steady-state test monitors engine coolant temperature drop during engine operation. The described rationality checks will detect coolant thermostat (coolant temperature below thermostat regulating temperature) failures.