5 Symptoms Of A Bad Car Thermostat (& Replacement Cost)

Unusual Temperature Readings

Temperature Gauge
Temperature Gauge

When you start up your vehicle after a long rest, you will usually notice that the temperature gauge starts out from the cold side and eventually rise up to the mid, which is actually the optimum engine temperature. However, if there’s any malfunction with the thermostat, the coolant flow to the radiator will be affected. This means that if the thermostat is stuck closed, the temperature will continue to rise dangerously. While if the thermostat is stuck open, the temperature will likely stop before reaching the mid-point.

Checkout: Why is your Car Overheating? Vehicle Cooling System Explained

How Long Does A Thermostat Last?

A thermostat is going to be working whenever you start your car. And it is going to open and closed millions of times when you have used your car for a while. When considering this repeated motion of a thermostat, it is definitely going to bring up the question about the lifespan of a thermostat.

Even though a thermostat is going to go through this kind of motion constantly, generally it is not going to break regularly. Thermostat failures do not happen that often but it is not unheard of either.

It’s hard to put a number on how many years it is going to last but usually, it is going to work fine for about 10 years. Unless you suspect a failure with the thermostat, there is no reason to change it. If you are doing a major repair with your car’s cooling system, it is a good idea to swap out your thermostat if you think it has seen better days.


Can You Drive Without a Thermostat?

A short term solution to a thermostat that is stuck closed is to remove the thermostat entirely.

While it is possible to drive without a thermostat, it is not wise to do so long term. Engines run very poorly before they reach operating temperature.

If you are missing your thermostat, your engine will probably not be able to reach operating temperature (unless you are running your car hard on a race track, and it’s blisteringly hot outside).

Symptoms of a Failing Car Thermostat

Fortunately, a thermostat failure is one of the many car problems your can diagnose and fix yourself. Here are the signs your car thermostat is failing:

  • The temperature gauge reads high and the engine overheats.
  • The temperature changes erratically.
  • The vehicle’s coolant leaks around the thermostat or under the vehicle.

Engine Overheating

Here’s how thermostats work. The manufacturers inject a mixture of ground-up brass and wax into a copper cup called a pellet. Then they slip a highly-polished metal rod through a rubber “O-ring” gasket and into the wax. The pellet is sealed with a crimp ring.

Next, a metal “skirt” is welded around the pellet. When the engine is cold, a spring forces the skirt up against a seat (just like a closed faucet), stopping the flow of coolant. As the engine heats up, the wax melts and expands.

Expansion pressure builds to the point where the wax tries to “spit out” the metal rod. But the rod can’t go anywhere. It’s attached to a “bridge” on the other side of the thermostat.

Eventually the pellet itself moves, overcoming the spring’s pressure. So the thermostat opens and allows coolant to flow. The entire system works well until the metal rod corrodes. That corroded rod damages the rubber seal and the wax leaks out. Once that happens, the thermostat stops opening, coolant stops flowing and your engine overheats. The result can be catastrophic engine failure costing several thousand dollars.

Erratic Temperature Changes

Keep an eye on your dashboard’d temperature gauge. If the needle spikes and drops, a thermostat failure may be to blame.

Coolant Leaking

Check the thermostat housing for leaked fluids. The coolant helps to keep your engine functioning at the right temperature, and the lack of fluid can affect its performance.

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.

Strange Noises From The Engine Bay

Noises From The Engine Bay
Noises From The Engine Bay

If temperature changes weren’t enough, you will also experience strange noises from within the engine bay. These noises can arise from within the radiator, or the engine or even both. Resembling a knocking, boiling or gurgling sound, these strange noises can indicate a failed thermostat and a lot many problems in addition to it.

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.

Final Thought

At this point, you must have known the symptoms of a stuck open thermostat, symptoms of a bad thermostat, and how to fix bad thermostat problems.

You might notice just one or two of the outlined symptoms when your thermostat goes bad. So, once you notice any of these symptoms, you want to diagnose and replace a bad thermostat, or better still, contact a professional mechanic for proper inspection and repair.

Bad Thermostat Symptoms

When your car thermostat fails, it will impair the operation and functionality of your engine. Fortunately, there will be some easily recognizable symptoms that you will notice when this happens.

You should take it upon yourself to replace your thermostat promptly if it turns out that it’s faulty or develops a leak. If you don’t do this promptly, then your engine could suffer irreversible damage the longer it is overheated.

Below are 5 of the most common signs that will indicate a faulty thermostat in your vehicle.

See Also: Symptoms of a Bad MAP Sensor

#1 – Temperature Gauge Reading Higher (or Lower) Than Normal

When you first start your car, the needle in the t

When you first start your car, the needle in the temperature gauge should be in the Cold side if the car has been sitting for a while. As you’re driving, you’ll normally notice the temperature gauge gradually creeps up until it reaches about the midway point in the gauge, which is the optimal engine operating temperature.

In the situation where the thermostat is stuck closed, it will prevent coolant from flowing into the engine. This means that your temperature gauge will continue rising until it’s all the way to the Hot end of the gauge. If you continue driving your vehicle at that point, you increase your chances of engine damage the longer you drive.

This is why it’s important to always keep an eye on the engine temperature gauge. As soon as you notice the temperature going higher than it normally does, it’s a good idea to pull over to allow the engine to cool down.

In cases where the thermostat is stuck open, you’ll notice that the temperature needle moves up much slower than usual and likely stops before it gets to its normal mid-point on the gauge. If you notice this, try cranking up your heater and if it doesn’t blow warm air out of the vents, you’ve pretty much confirmed a broken thermostat.

#2 – Sudden Air Temperature Changes Inside Vehicle

One symptom that you can often feel is when the ai

One symptom that you can often feel is when the air temperature inside your vehicle suddenly changes. It may start off by dropping to a very low temperature and then suddenly spike up to a very high temperature.

Any change in air temperature that doesn’t reflect your current HVAC settings will often mean that something is wrong with the thermostat.

#3 – Coolant Leaking

A faulty thermostat most likely causes it to remai

A faulty thermostat most likely causes it to remain in its closed position. This means when the engine is running hot and the coolant normally flows toward it, the closed thermostat will block the coolant from going into the engine. This will cause the coolant to overflow from the thermostat housing.

If it stays like this and you don’t do anything to fix it, your coolant hoses will be the next things that leak. The quickest way to see if you’re leaking coolant is to just check under your vehicle. If you see green-ish or red-ish liquid leaking out of your vehicle and staining the surface underneath, then you definitely have a coolant leakage situation.

#4 – Rumbling Noises

If temperature changes were not bad enough, you wi

If temperature changes were not bad enough, you will begin hearing strange rumbling noises as well. These noises will be coming from your radiator, engine, or both. The noise could also resemble a knocking sound, boiling sound, or gurgling sound as well.

Basically, if you’re hearing strange noises as described above and you experience one of the other symptoms listed here, then you most certainly have a thermostat problem.

#5 – Heater Malfunction

If you live in a cooler climate or are driving aro

If you live in a cooler climate or are driving around in the middle of winter, you may an opposite temperature problem in your vehicle. Instead of being in a stuck closed position, it will be stuck open.

This means it will keep letting coolant flow into the engine, even when the engine doesn’t need it. So if you turn the heater on inside the vehicle cabin and thermostat is open, then cool air will continue to come out of the HVAC vents even when you turn up the heater temp all the way. You won’t get the heat that you want to warm yourself up.

How to change or test a thermostat

Testing (and, if needed, changing) a thermostat is not difficult, but there are several important precautions to take. First, never crack open your car’s cooling system if the engine is hot; that’s a good way of getting scarred for life. Second, as mentioned above, the housing is sealed by a gasket, which we strongly recommend you don’t re-use — it may not even come out in one piece. Order the correct gasket (and check if it needs sealant as well), wait for the engine to cool down, keep rags handy, and this shouldn’t take more than a few minutes of your time.

On most cars, the thermostat is located near the top of the engine. That’s not always the case: if you drive a Subaru, you may need to crawl under the car to access it. After placing a pan underneath where you’ll be working, remove the hose going to the thermostat housing, loosen the bolts that secure the top part of the housing, take it off (and use a rag to wipe off any coolant that drips), and you’ll see the thermostat. Remove it and carefully scrape off any gasket residue left. If you know that the thermostat is bad, pop the new one back in (make sure it’s in the right direction), fit the new gasket, tighten the bolts, re-install the hose, and you’re good to go. If you’re not sure, head to the kitchen.

If you know how to make pasta, you know how to test a thermostat — just don’t add salt. The main difference is that, when you’re cooking fettuccine you add the pasta after the water starts to boil. When you’re testing a thermostat, you’ll want to drop it in a pan of cold water and then bring it to a boil. We’re assuming that your thermostat is cold and therefore in the closed position; if it’s cold and it’s open you’ve found your problem. Place it in a pan full of water, wait until it boils, and use a ladle to fish it out. It should be in the open position; if it’s not, it’s stuck closed. Wash the pan thoroughly after testing a thermostat, otherwise your next arrabbiata sauce is going to taste a little funky.