The leading cause of a car thermostat going bad is corrosion from coolant not being changed regularly. By following your vehicle's recommended maintenance schedule, you can ensure that your engine cooling system stays adequately serviced. By neglecting your cooling system, old coolant can become worn out and start to degrade your thermostat, water pump, and radiator, releasing small particles throughout your cooling system. A corroded thermostat can become stuck in a closed position, causing your engine to overheat.