A fail-safe thermostat protects the engine in a vehicle when it overheats. It remains in the open position to provide maximum coolant flow so that the engine is able to cool down.
Standard thermostats lock into the closed position when overheating occurs, which damages the engine by not allowing coolant to flow to the engine. A fail-safe thermostat works like a standard thermostat under normal operating temperatures.
When overheating occurs, the fail-safe thermostat has a secondary stroke that activates a piston to lock the thermostat in the open position. This gives the owner time to pull over or reach his destination with minimal damage to the engine. Once the thermostat has been locked open, it must be replaced with a new one.