- The thermostat's function is to monitor engine temperature and regulate the flow of engine coolant through the engine and radiator to avoid overheating. A small valve in the thermostat is regulated by a temperature-reading device. When the device detects that the engine is warming up, it opens the valve slightly, allowing the warming engine coolant to seep out of the engine and into the radiator, where it is cooled and then pumped back into the engine. The warmer the engine gets, the wider the valve opens. Most thermostats fail in one of two ways---either the temperature-reading device malfunctions, or the valve malfunctions.
- The temperature gauge often offers the earliest warnings of a failing thermostat. The temperature data monitored by the thermostat is communicated to a temperature gauge on the car's instrument panel. This gauge is usually a needle that moves along a spectrum from "H" (hot) to "C" (cold), and often has a symbol of a thermometer next to it. When you start a car with a cold engine, the needle starts at "C" and gradually rises to the middle of the spectrum as the car warms up. If you notice that the needle barely rises or doesn't rise at all after driving several miles, it's an indication that the temperature-reading device in the thermostat has failed. If you notice that the needle is climbing close to "H", that's your warning that your car is about to overheat. This could be caused by a stuck thermostat valve or another failed thermostat component, but it could also be caused by low engine coolant levels, a leaky hose or a cracked radiator. If this happens, you should pull over immediately and let the engine cool before trying to start it again, then drive directly to a service center if possible.
- A failing thermostat can cause several other issues that you may notice before the problem is reflected in the temperature gauge. Since a failing thermostat can cause an engine to run too hot, you may notice diminished gas mileage. If you're running your car's heater and notice that the air is slow to heat up or never heats up, this is another sign of thermostat failure. The buildup of heat can also cause strange engine noises like loud knocking or a metallic pinging sound. You may also hear the engine idling at high speeds or notice that your RPM gauge is running at around 900 RPM or higher. If you can normally hear your engine cooling fan and notice that it is running continuously, that may also indicate a problem.
- Unfortunately, it is possible for a thermostat to fail without exhibiting any of the more obvious symptoms. If the temperature-reading device and the valve both fail simultaneously, your engine temperature may rise to dangerous levels and your temperature gauge may never report the danger, causing your engine to overheat unexpectedly.
