5 Symptoms Of A Bad Or Failing Thermostat

A healthy engine should run above 200 degrees. The radiator cooling fan on some vehicles doesn’t even engage until after the coolant reaches nearly 230 degrees. Although most modern engines are expected to run 200-230°F, most thermostats open at just under 200°F.

Disregarding a bad thermostat may result in catastrophic engine failure, so you must replace it as soon as it begins to show signs of trouble. Fortunately, you can determine when it’s time to get a replacement by watching for signs of a bad thermostat.

What Does a Thermostat Do?

Your car’s thermostat is a vital component that is actually pretty simple. It’s a valve located in your car’s cooling system. Its job is to regulate the amount of coolant that is recirculated back into the engine and how much is cooled via the radiator prior to being recirculated.

This ensures that the coolant in your engine stays within a specific temperature range regardless of the outside temperature. The valve is opened and closed with a spring, piston or another thermally controlled device.

Key Points:

  • The thermostat is a valve that regulates coolant temperature, keeping it in the best range for the engine.
  • If the engine runs too hot or cool, you might have a bad thermostat on your hands.
  • You can check a thermostat for problems by examining the upper radiator hose’s temperature after the engine has warmed up.
  • You should replace a bad thermostat immediately and avoid driving without one until you get a replacement part.

Problems A Thermostat Can Experience

The thermostat isn’t a maintenance or wear-related item that requires replacement at any pre-set level. Instead, it’s usually only replaced if failing, or whenever the water pump is replaced. The main problems a thermostat can experience are being stuck open, stuck closed, or leaking.

1. Thermostat Stuck Open

If the thermostat is stuck open, an optimal engine temperature is never going to be reached, and there will be no or little heat available in the passenger compartment, and engine wear can increase.

2. Thermostat Stuck Closed

If it’s stuck closed, the engine is going to overheat because coolant isn’t moving to the radiator. Debris or a broken spring in the thermostat could be responsible for it being stuck in either the open or closed position.

3. Leaking

Finally, the thermostat housing can fail and begin leaking coolant.

Signs Of A Bad Thermostat

The thermostat is responsible for regulating coolant flow through the engine. If the engine is cold, it stays closed to let it warm up. As the engine heats up, the thermostat gradually opens, allowing coolant to reach the radiator. Whenever it begins to fail, the engine overheats and, if the problem is left unchecked, leads to your engine boiling over. This can lead to costly repairs.

Signs Of A Bad Thermostat

While relatively cheap, a thermostat is one of the most crucial car parts. If you don’t want to change your engine entirely, you must address the root issue with your failing thermostat. You’ll need to look out for the following signs:

  • Overheating (or overcooling)
  • Coolant leaking
  • Erratic temperature changes
  • Strange sounds
  • Heater problems

#1. Overheating And Overcooling

Overheating is the most common symptom of a failing thermostat. Due to corrosion or aging, your car’s thermostat can get stuck in a closed position. If this happens, the thermostat will not let the coolant reach the radiator, and, as mentioned above, the engine will overheat, causing severe damage.

One of the lesser-known issues is the polar opposite of overheating. Overcooling happens when the engine does not reach the right temperature for its normal function. In this scenario, the thermostat is stuck open and overflows the radiator with coolant, drastically reducing the engine life cycle.

To prevent this, you should always watch your dashboard temperature gauge. If you don’t have one, monitor various warning lights in the instrument cluster. If you find yourself in a sticky road situation, try turning the heater on full blast to dissipate excess heat.

#2. Coolant Leaking

If the thermostat fails in its closed position, then this can cause the coolant to overflow and leak out of the thermostat container when the engine overheats. If you find coolant leakage, it could be a sign that the thermostat has broken and requires replacement.

The easiest way to see whether your vehicle is leaking coolant is to check underneath the vehicle. If you find reddish or green-ish appearing liquid staining the ground, you probably have a coolant leakage problem.

If you avoid getting this problem fixed, then you will begin to have a leaking problem with the coolant hoses, and this could continue to affect other related parts.

#3. Strange Sounds and Temperature Changes

Another symptom of a faulty thermostat is bizarre sounds. You may hear rumbling, boiling, or knocking coming from the radiator. All these signs point to an issue with your car’s cooling system.

The thermostat also affects the temperature of air that flows in your vehicle through the vents. If it suddenly changes from hot to cold or cold to hot, this is a clear sign of faultiness, especially if it happens erratically.

#4. Increased Fuel Consumption

Both engine overheating and engine overcooling can largely increase exhaust emissions and damage fuel economy. This happens because the vehicle is not able to reach its operating temperature.

In this case, thermostat failure symptoms will most probably be translated into an increased fuel consumption and a resulting unexpected spike in monthly utility bills.

#5. Heater Problems

If driving during the winter or in a cooler climate, your thermostat may get stuck in an open position which will affect the heating system. If stuck open, the thermostat will allow coolant to continue to flow into the engine, even when it doesn’t require more coolant.

If this happens, and you turn the heater on simultaneously, then only cool air will come out of the vents, no matter how high you turn up the heat.

In cases where you don’t require hot air blowing out the vents, you can still check to see if the thermostat is stuck open. Look at the needle on the thermostat gauge and see whether it moves slowly before it stops before the normal point on the gauge.

If this is the case, then crank up the heater to see if it blows hot or cold air. If it blows cold air, then most likely the thermostat is broken.

Because so many other engine parts rely on the thermostat’s functionality and readings, you should be sure to get the thermostat looked at or replaced if you think it might be faulty. Otherwise, your engine could suffer irreversible damage and other serious problems.

Related Posts: How To Replace A Bad Thermostat In Your Car?

Can I drive with a thermostat problem?

If the thermostat is stuck closed, no, you should stop driving immediately; if you don’t, the engine can quickly overheat. An overheating engine should be shut off, and the vehicle should be towed. Overheating can severely damage the engine after only a few minutes.

If the thermostat is stuck open it may not cause engine failure, but you’ll likely notice poor performance, an ineffective heater and bad fuel economy. It should be replaced at your earliest convenience.


How does a car act when the thermostat is bad?

Overheating is the most common symptom of a failing thermostat. Due to corrosion or aging, your car’s thermostat can get stuck in a closed position. If this happens, the thermostat will not let the coolant reach the radiator, and, as mentioned above, the engine will overheat, causing severe damage.

How can I tell if my thermostat is bad?

A bad thermostat can be detected when your AC or furnace is not turning on, your HVAC systems are not turning off, the thermostat not responding, your HVAC system is short-cycling, the room temperature not matching the temperature displayed on the thermostat, and the programmed setting resetting themselves.

How do I know if my car needs a new thermostat?

Signs My Car’s Thermostat Is Going Bad or Has Failed Completely:
1. Odd Temperature Gauge Readings.
2. Air Temperature Changes.
3. Coolant Leaks.
4. Strange Sounds.
5. Heater Problems.

How do I know if its the thermostat or water pump?

If your car is overheating, it could be a sign of a faulty thermostat. However, if you notice coolant leaking from the front of the engine or a whining noise coming from the front of the engine, it could indicate a problem with the water pump.

How much does it cost to fix a car thermostat?

Generally, repairing a thermostat costs around $200 to $500. This is the price range you’ll find for most vehicles, although some may be higher or lower depending on the circumstances. But if we’re talking complete replacement, expect the price to increase by $50.