What are the signs of a bad thermostate in a car?

Signs of a bad thermostat in a car include the engine overheating or running too cold, fluctuating temperature gauge readings, and poor heater performance. You may also notice coolant leaks or a noticeable decrease in fuel efficiency. If the thermostat is stuck open, it can prevent the engine from reaching optimal operating temperature, while a stuck closed thermostat can cause overheating. Regularly monitoring these symptoms can help identify thermostat issues early.