Why Car Thermostats Failby Michael Ryan
Car thermostats fail when the valve of the thermostat fails to open, fails to close or gets stuck in a partially-open position. As the thermostat controls the flow of coolant to the engine, this can keep the car from warming up or cause it to overheat.
Like all items on a car, the thermostat wears a little bit with each use. The continual opening and closing of the thermostat will eventually cause it to wear out.
The most problematic type of failure is a thermostat failing closed. By keeping coolant from entering the engine, the engine will overheat. If a thermostat fails as it is stuck partially or completely open, the vehicle may be unable to reach its full operating temperature, which uses more fuel and can prevent the cabin heater from reaching full heat.
A thermostat that fails closed will cause the temperature gauge to rise until the car overheats. You may see smoke from under the hood. A thermostat that fails open or partially open is identifiable by a temperature gauge that does not rise to its normal operating point, normally halfway between the low and high temperature marks.
A thermostat that is stuck open requires service, just not as immediate as one that has failed closed. A permanently open thermostat will allow coolant into the engine from the moment it is turned on, which will keep the engine from warming up.
If you want to avoid having a thermostat fail in the closed position, some manufacturers offer thermostats that are fail safe, meaning that they will be permanently stuck in the open position if there is a technical problem. This can provide peace of mind if the thermostat fails while you're on a trip.