How do you tell if your BMW has a bad thermostat? Thermostats fail in one of two ways; they either fail stuck open or fail stuck closed. When they fail in a closed fashion its usually easy to tell because your car will likely experience severe overheating problems with the temperature needle buried in the red. This is the most dangerous type of failure because an overheating engine can cause major damage, including head gasket failure and cracks in radiators and cooling system components. In more severe cases, head bolts can strip the weak threads in the aluminum block of an M54 model engine.