Thermostat
Definition
The thermostat is a wax-element valve that blocks coolant flow to the radiator until the engine reaches operating temperature (usually 85-95 °C), then opens progressively to hold it there; map-controlled versions have an electric heater so the ECU can shift the opening point. One stuck open causes slow warm-up, poor heater output, high consumption and code P0128, since the ECU never sees regulating temperature; one stuck closed causes rapid overheating with a cold radiator hose. Plastic housings crack and leak at the seams, and electric variants fail their heater circuit. Verify by watching coolant temperature on live data: it should climb steadily, plateau at the rated value and the radiator inlet hose should get hot right at that point. Codes: P0128 (below thermostat regulating temperature), P0597-P0599 (thermostat heater control circuit), P0116/P0117 if the sensor is implicated.