Several things can happen to an automobile that can make you think the thermostat has failed, but that is not always the case. A closer inspection of the car will show you the differences between a failed thermostat and a different and possibly more difficult repair. However, if you have noticed that your car's cooling system just isn't acting right, it's time to diagnose the issue.
Eliminate the fan relay as a possible problem by reviewing any trouble codes that the vehicle has stored in the computer. If the fan relay has broken, the fan will not turn on. The car will then show or store a trouble code. Trouble code readers can be expensive if you do not already have one on hand. You can also warm the engine up for 10 minutes and watch to see if the fan turns on. If it does not, the relay is likely the issue, not the thermostat.
Turn the engine on and turn the heat and fan blower on full blast. Check the inside of the windshield for a greasy fog. You will be able to tell by running your finger across the windshield. A greasy fog means the heater core has ruptured internally and it is spraying evaporated radiator fluid. You can also tell by the distinctive sweet smell of radiator fluid. If you are not familiar with the smell of radiator fluid you can remove the radiator overflow cap--after the engine has cooled-- and determine what it smells like. If this is the case, the heater core must be replaced.
Check the floor boards for radiator fluid. Depending on the brand of vehicle you have, the heater core is located on either side of the vehicle. Any radiator fluid on the floorboards of the vehicle means the radiator on the heater core or the hose has burst and the heart core is pushing fluid onto the floorboards. The heater core must be replaced or repaired.
Start the engine and turn the heat and fan blower on full blast. Allow the engine to warm up for 10 to 15 minutes. If the blower blows cool air or slightly warm air, the thermostat must be replaced. Thermostats are designed to fail in the open position. This means fluid will run unrestricted through the engine, thus not allowing the engine to heat properly. The result will be a lack of warm air inside of the car.
Start the engine and allow it to run for 10 to 15 minutes. If the radiator fan comes on and the engine overheats immediately, shut the car off. Have the engine cooling system professionally flushed out to unblock any clogged coolant veins within the engine block. Engine sludge can block the engine's ability to pass fluid through the veins. This will cause the engine to overheat.
In some cases engine sludge can cause the thermostat to fail in the closed position. If it's determined that the engine must be flushed out to clear sludge, many people will change the thermostat for good measure.