What if your 2004 Audi a8 temperature sensor is going over 90 degrees to 110 even to 120 but when you turn the heat on the sensor goes back to 90 the mechanic is saying to change the thermostat?

If your 2004 Audi A8's temperature sensor is reading unusually high (90 to 120 degrees) but drops to 90 when you turn on the heat, it suggests that the engine is overheating and the coolant is not circulating properly. This behavior often indicates a malfunctioning thermostat, which may be stuck closed, preventing coolant flow. Replacing the thermostat is a reasonable step to take, as it can help restore proper temperature regulation. However, it's also wise to check for other potential issues, such as low coolant levels or a failing water pump.