Your thermostat is stuck and not regulating engine temperature properly, like a broken valve that won't open or close. The engine either runs too hot or too cold because coolant isn't flowing correctly through the radiator.

The ECM monitors coolant temperature via the coolant temperature sensor and expects the thermostat to maintain temperature within a narrow operating window. When coolant temp rises above or falls below expected ranges for given engine load conditions, the ECM detects thermostat malfunction. The system uses response time and stability of temperature readings to validate thermostat operation.

Voltage & Parameter Thresholds