Car thermostats have long lives, but they do not last forever. When they fail, you will get a lesson in how important a very small part can be. A failing thermostat will always bring symptoms along with it that most people will not enjoy. With 5 years as a mechanic, I have seen plenty of car thermostats fail.
There may be a lack of heat in cabin.
One of the jobs that thermostats do very well is getting your car up to the proper operating temperature. At temperatures between 180 and 200 degrees Fahrenheit, the circulating coolant will let your car’s heater keep the cabin occupants nice and toasty most of the time. When the thermostat sticks open, the engine will not heat up properly. This causes the air being circulated to the passengers to be too cool for comfort in cold weather. Many times, this can start as an intermittent problem because the thermostat will work sometimes and fail sometimes. Eventually, it will fail completely.
Car overheats for no apparent reason.
The opposite of the thermostat sticking open is that it will stick shut. As would be expected, the result is also the opposite. Instead of the engine running too cool, it will overheat. After you have eliminated leaky hoses and other possible leaks, the thermostat is the most likely culprit. When the thermostat fails to open, your car will overheat fairly rapidly after you start driving. Usually, a two or three miles of highway speeds will send your temperature gauge into the red zone or turn on the hot engine light.
Coolant does not circulate when car is warmed up with radiator cap removed.
Let the car cool off. Remove the radiator cap. Start the engine and watch the coolant as the car warms up. You should be able to see it moving or circulating through the reservoir at the top of the radiator. If there is no lateral movement, this is a good indicator that it is time for a new thermostat. Also, the coolant may rise and flow out of fill spout when cap is left off as car warms up. This happens because the coolant cannot circulate, and it is expanding as it warms up.
Leaks form in radiator or heater hoses.
If your motor is overheating frequently, the extra pressure in the system can put lots of stress on the hoses or in the part of the radiator that does the cooling. This pressure can cause weak areas to fail and start to leak. Not every cooling system leak is a thermostat problem, but a bad thermostat can lead to leaks.
Unknown cause of low coolant level.
This pressure from overheating can also cause coolant to be forced up and around the radiator cap. It is almost impossible to detect this type of leak except that your coolant will keep getting low for no apparent reason. If this happens too often, you may want to consider replacing the thermostat because it is a relatively cheap and quick job.