Symptoms of a Bad or Failing Thermostat

A car thermostat regulates the flow of coolant through the engine and is an incredibly important player in the operation of your vehicle’s engine. You may hear the phrase “the thermostat was stuck open or closed.” When the engine has been sitting for a while and is not warm, the thermostat will be closed. Once the engine is running and reaches a certain operating temperature, a sensor inside the thermostat will cause it to open, allowing coolant to flow to and from the radiator, decreasing the temperature so it can be recirculated through the engine again. This constant flow (in conjunction with several other cooling system components) keeps your vehicle’s engine functioning at the optimum temperature.

The opening and closing of the thermostat at the correct time are critical to maintaining proper engine temperature. In the event the thermostat happens to become “stuck” closed, there is no way for coolant to be circulated through the radiator and eventually back through the engine, which causes extremely hot engine temperatures. Likewise, if the thermostat is “stuck” open, the flow of coolant is constant, it results in the vehicle’s engine temperature never reaching an optimum level of heat, creating performance problems and accelerating the wear-down of parts. There are 4 common symptoms associated with a bad or failing thermostat.

1. High-Temperature Reading and Engine Overheating

The first and potentially most alarming symptom will be the temperature gauge reading high into the red within the first 15 minutes of your vehicle engine running. This is often the first sign that the thermostat is not functioning properly. It means coolant isn’t entering the engine because the thermostat is stuck closed, and your car can suffer engine damage fast.

2. Low-Temperature Reading and Underheated Engine

A thermostat stuck in the open position constantly pushes coolant into the engine and causes a lower operating temperature. Your temperature gauge will show a needle that barely increases or remains at the lowest setting. This will reduce engine efficiency and increase emisssions over time, as well as accelerate the deterioration of parts.

3. Temperature Changing Erratically

Erratic temperature fluctuations can also occur, causing dramatic spikes and drops in temperature that eventually lead to poor engine performance and reduced gas mileage. In this case, you may see the temperature abnormally low at one point and, shortly after, climb to an abnormally high level. The thermostat itself isn’t stuck in any one position, but it will still produce false readings and cause issues with coolant regulation.

4. Coolant Leaks Around Thermostat Housing or Under Vehicle

Another indication may be leaking coolant, which can occur when the thermostat does not allow coolant to flow when stuck in the closed position. This can be noticeable in a variety of locations, but most commonly around the thermostat housing. It can eventually cause other coolant hoses to leak as well, often resulting in coolant leaking on the ground under your vehicle.

Thermostat replacement is a fairly inexpensive repair to make to your vehicle, preventing potentially thousands of dollars in engine damage due to excessive heat. If any of the above symptoms sound familiar to you, it may be time to have an expert mechanic diagnose your vehicle.


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