Temperature Gauge

• Paying attention to the temperature gauge on the dashboard of the car is important when you suspect a potentially malfunctioning thermostat. If you notice odd swings on the temperature gauge or halting movements, this is often caused by a failing thermostat -- this condition manifests itself as a stop-and-start on the gauge, with sudden, wild swings into the higher regions.
  
  

Overheating

• If the vehicle continually overheats and you've eliminated a malfunctioning temperature sensor, the cooling fan and its sensor, the radiator, the water pump as causes, and none of the hoses show any sign of leaking, then the last place to look is the thermostat. Sometimes if the car experiences a few episodes of overheating due to low coolant or a ill-fitting radiator cap, this can cause the thermostat to fail. Overheating may ruin the thermostat's ability to open or close at the preset temperature for the vehicle.
  
  

Gas Mileage Drop

• If you notice a gas mileage drop and no heat coming from the heater, this is a good indication that the thermostat is stuck in the open position. While thermostats don't just fail, over time they can lose their ability to respond to the temperatures of the engine and stick in the open position. This causes gas to not burn fully and to condense in the cylinders, resulting in a drop in gas mileage.
  
  

Removing and Replacing the Thermostat

• Most thermostats sit inside a metal housing on the engine, attaching to an inlet hose coming directly from the radiator. With its rounded shape and two bolts that hold it to the engine, it's relatively easy to spot. Replacing it is quick; the hardest part, depending on the vehicle model, is reaching it and removing the bolts that hold it in place. Because it is subject to continual heating and cooling, the expansion and contraction often make the bolts hard to remove. Add an extender to your ratchet to unscrew the bolts more easily. Remove the housing, scrape off any gasket material, add a new gasket and sealer and replace the thermostat. There is only one way it installs correctly, so ensure you don't reverse it, or the thermostat won't work right. Some engines have a line-up notch. There is often an arrow which points in the direction of opening and water flow.