A vehicle's thermostat is partially responsible for maintaining proper engine temperature. It's designed to control the flow of coolant through the engine. If it's not opening and closing properly, your vehicle may display several different problems. Though your car is better off with the thermostat stuck open rather than closed, you should repair the component immediately if you suspect it is stuck open.
Low Engine Temperature
If your car is running unusually cool and not building up any heat in the engine, it's likely that your thermostat is stuck open and is letting too much coolant through. A thermostat that is stuck in the closed position will cause your vehicle to overheat. Most drivers know when the vehicle's engine temperature gauge creeps toward the red area but may not be aware that an engine is running cool. It's important to know your engine's regular operating temperature because running too hot or too cold are both signs of problems with the cooling system.
Your vehicle's heater works in conjunction with the engine's cooling system. The heater builds up warmth as the vehicle runs and brings in the warm air from the engine to warm the interior of the car. If the engine does not warm up enough because the thermostat is stuck open, the heater will not deliver warm air to the interior.
Reduced Fuel Economy
When your engine runs cooler than it should, it can cause your fuel to condense on your engine's cylinders because of the lower water jacket temperature. If your fuel becomes condensed, it will not burn like it should and your car will get slightly worse gas mileage than it did before the thermostat got stuck in the open position.