The 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.
A thermostat that is
stuck closed will cause the vehicle to overheat. If a new thermostat doesn't resolve engine temperature problems, the cooling system needs to be checked for other issues.
» LEARN MORE: Signs of a bad thermostat, and how to fix it