If you’re experiencing an overheating engine in your BMW, one of the culprits might be the thermostat. A bad thermostat can cause two different sets of problems, depending on whether it's stuck open or stuck closed. A thermostat that is stuck open can cause the engine to run colder than normal and turn on the check engine light. It may also cause poor fuel mileage and the heater to blow cool air. These are NLA through BMW!