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Symptoms of a Failing Car Thermostat (Explained)

Updated: Jan 9

Your car's thermostat regulates the coolant flow through the engine and is a crucial part of how it works. The most frequent cause of overheating or a lack of heat in your car is a faulty thermostat.

The most common symptom of a failing car thermostat is overheating. Your car's thermostat may become stuck in the closed position due to corrosion or age. A car thermostat stuck closed will prevent the flow of coolant to the radiator, causing the engine to overheat.

Serious damage can occur as a result of a failing thermostat and overheating, such as engine failure, blown head gasket, cylinder head warping, cracked liners, etc.

However, overheating is not always the result of a faulty thermostat; there are other indicators of a failed car thermostat, as listed below, that you should look for to help diagnose the problem.


Table of Contents:

failing car thermostat

Symptoms of a Failing Thermostat

A thermostat failure is fairly easy to diagnose. Listed below are the various symptoms of a malfunctioning car thermostat.

  • Overheating

  • Changes in cabin temperature

  • High and fluctuating coolant temperature

  • Thermostat coolant leakage and low coolant

  • Unusual sounds and steam from engine bay


When the engine is cold, the thermostat is closed, allowing the coolant to circulate the engine and quickly warm up to operating temperature.

When the engine reaches working temperature, the thermostat opens, allowing coolant to flow to the radiator and regulate coolant temperature.

The system is in good working order until the metal rod inside the thermostat starts to rust. If this happens, wax inside the thermostat leaks out, causing the thermostat to stop working as it should.

Your engine will overheat if the thermostat stops opening and the coolant stops flowing to the radiator. The result might be a catastrophic engine failure.

Changes in Cabin Temperature

This is a common sign of thermostat failure when the temperature inside your car abruptly changes. It might start at a very low temperature before rapidly increasing to a very high temperature.

Any change in air temperature that is not compatible with your current interior climate settings can show that the thermostat is not working properly.

High & Fluctuating Coolant Temperature

Keep an eye on your dashboard's temperature gauge. If the needle spikes and drops while driving, it might be due to a thermostat failure.

It should be noted that some temperature changes are common during regular operation, but they should be very slight; if the thermostat is malfunctioning, there may be large dips and spikes.

The coolant temperature can sometimes become very high and then return to normal, only to rise again.

Thermostat Coolant Leakage & Low Coolant

Check for coolant leaks around the thermostat, if there is leakage it can be a sign of thermostat failure.

Ensure you check your coolant levels too, if there is a leak it can lead to low coolant and can cause overheating.

car thermostat

Unusual Sounds & Steam From Engine Bay

Rumbling, hammering, boiling, or bubbling sounds may begin to develop. These noises will be produced by your radiator, engine, or both.

If you're hearing unusual sounds like the ones listed above and are also noticing other signs, your car could have a thermostat malfunction.

You may also hear a hissing sound, which might be produced by very hot coolant evaporating and high coolant pressure. This coolant may then force its way out as steam through tiny gaps in the coolant system, such as the coolant filler cap.

How to Check a Car Thermostat

Before removing the thermostat from your vehicle, check the coolant flow and temperature.

  • After removing the radiator cap, idle the engine. If the coolant flows instantly, your thermostat is stuck open.

  • If coolant isn't flowing, wait till it warms up. The coolant should reach the right temperature and begin to flow after 10-20 minutes.

If your thermostat does not start to flow but the temperature gauge on your dashboard rises, it is stuck closed. If your coolant is always flowing, there might be another cause for your engine overheating.

To check the temperature of your coolant, start with a cold radiator and engine. Idle the engine and check the temperature of the engine block or cylinder heads with a thermometer. The top radiator hose's temperature should then be tested.

Repeat the test after five minutes. Complete the test three times in total.

  • If the temperature does not rise significantly, your thermostat is stuck open.

  • If the temperature of the radiator hose remains constant but the dashboard gauge rises significantly, your thermostat is stuck shut.

  • If the top radiator hose reaches a temperature comparable to the engine block, your thermostat is allowing sufficient coolant flow. It's likely that your cooling issues are caused by something else.

To test the thermostat outside of the car, place it in a pot of water on your kitchen stove. Allow the thermostat to come into touch with the bottom of the pot. Use a thermometer to determine the temperature at which the thermostat opens.

Take note of the temperature when it first begins to open and when it is completely open. Remove it from the heat. Keep an eye on it as it cools to see how it gradually closes. 

Replacing the Faulty Thermostat

If your thermostat fails, don't just replace it and refill the coolant reservoir. It's alerting you to another serious problem; the corrosion inhibitors in your coolant have also failed. When replacing the thermostat, clean the coolant system and replace the coolant.

Also, don't try to save money by selecting a low-cost thermostat. Premium thermostats offer better corrosion resistance.

Several manufacturers provide "fail-safe" variations that fail in the totally open position to prevent engine overheating.


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