Your car's thermostat helps the engine reach operating temperature more quickly by restricting coolant flow through the radiator. When your thermostat fails, you risk your engine becoming overheated and damaged. It can stick shut, in which case the ‘Engine Temp’ light will come on or a high temperature will show on your engine temperature gauge (though check that engine coolant isn’t down either). The approximate cost to replace a car thermostat by a paid mechanic can be over a 100 dollars, even though the part costs only about 10. Replacing a thermostat on a car is fairly easy. Check if the top hose has worn out and clean up the old sealant that stuck between the housing and the engine. Find and follow the upper radiator hose to where it connects to the thermostat housing which is where the opposite end of the upper radiator hose mounts to the engine. Drain the coolant from your radiator until it is below the thermostat so your anti-freeze doesn’t leak on to the ground. Loosen the clamp and remove the radiator hose from the thermostat housing and, from it, make sure you have towels or rags nearby to catch the anti-freeze that leaks. You need to remove the thermostat housing from the engine. You must first remove the bolts that hold this housing in place, remembering where the item is. Have a rag in hand and place it in the thermostat's mounting hole so that no gasket material can enter the engine and scrape the gasket from the housing and the engine. Reinstall the housing and make sure the bolts are locked in, then reinstall the radiator hose to thermostat housing and clamp. Refill the radiator with coolant and start your car. Allow your car to heat up to normal temperature; while it is warming up, check for leaks. If you discover any, tighten any bolt or lid as much as necessary. You can now stop your car, and allow it to rest completely. Check your coolant level, and add more if needed.