4 symptoms of a bad thermostat, possible causes & replacement costs

Updated Feb 05, 2021 | Same topic: Automotive FYIs

What is a car thermostat and how does it work? 

Today, most cars running around the world are propelled by internal combustion engines. With combustion being the key operative word for these machines, they generate a lot of heat.

As such, they require something to stave away the extra heat and keep the engine in operating temperatures. To do just this, cars are fitted with a thermostat.

How Thermostat Works

A car’s thermostat works by managing the coolant, which flows through the many nooks and crannies of an engine. It opens and closes depending on the temperature, which in turn is detected by sensors. If it's too hot, then the thermostat opens up, if it's too cold, the thermostat closes.

A picture of how a thermostat works

How a car thermostat looks when its closed or open [Photo Axleaddict]

That said, this part is particularly crucial to an engine’s life, and should be kept in working order. So, what are the symptoms of a broken car thermostat, or one that’s just about to go bad? We’ll list several, and we’ll also touch a bit on the estimated costs to get a car’s thermostat replaced. 

Symptom #1: High-temperature reading 

The most obvious symptom of a failing car thermostat is a high or consistently low temp gauge reading.

A picture of a temp guage reading at high temps

Seeing this early in the morning sucks

Often than not, this occurs around 15 to 30 minutes after starting the car. Sometimes, however, it can occur even before there are signs of overheating. Also, note that some more affordable car models actually lack a temp gauge.

>>> Related: Is it safe to use water as car radiator coolant? [Newbie Guide]


Symptom #2: Overheating engine 

Another obvious symptom of a bad thermostat is an overheating engine. For those who have not seen an engine overheating before, it’s going to be very obvious. It’ll produce lots of smoke/steam, the engine will lose performance, and you can even detect strange smells. 

A picture of a man inspecting his engine

If you're certain that your car's overheating, pull over to a safe spot

Symptom #3: Strange noises 

If the overheating isn’t as sudden as was described above. Then you might hear a gurgling, light rumbling sound coming from your engine bay. This means that your coolant, be in the one stored in a radiator or the coolant reservoir, is boiling away due to overheating.

>>> Related: What to do if your car overheats: 10 must-know things for Pinoy drivers

Symptom #4: Where’d my coolant go? 

Typically, your car’s coolant levels will gradually go up and down due to different engine temperatures. This is normal. If, however, your car’s coolant “magically” disappeared after just topping it up, then you might have a leak somewhere.

One of the probable places where this leak can happen is with the thermostat itself. This is caused when a thermostat valve is stuck shut and thus impeding coolant flow.  

A picture of a car puking coolant

The coolant that escaped your engine will typically make its way to the ground

Just to be certain, look around the area of the thermostat housing. If it is dripping wet, then you might have a coolant leak. Another simple way is to check for leaks on the ground after parking your car overnight.

Note that a car puking coolant in large quantities can be hazardous. This is because it contains ethylene glycol and methanol.

Maybe it’s just a sensor problem? 

As we’ve said above, a car’s thermostat needs sensors to work. That said, these symptoms might not stem from a faulty thermostat, but you might just have a broken coolant temperature sensor instead. This might be a cheaper fix, depending on the car model.

>>> Related: 10 Must-know Car Engine Sensors

How to replace a faulty car thermostat? 

Depending on the engine model, this might be difficult for some people. Most often than not though, this part is located in close vicinity to the car’s water pump on the cylinder head. The thermostat’s housing is linked to a radiator hose. As such, it can be easy to find and reach.

If you want to do this yourself, do read our guide on how to replace a bad car thermostat in six steps

Car thermostat replacement cost

Again, this depends on what kind of engine your car is running. For Honda’s the cheapest replacement thermostat that we can find is priced at Php 150. Genuine replacement thermostats can go from Php 800 to Php 1,600 above. Typically, it's more expensive for newer models. 

The most expensive replacement thermostat of course goes to cars that aren’t sold locally, those that aren’t common in the local automotive market, or high-end/high-performance cars.  

A picture of a car's thermostqat

A typical car thermostat

Take for example a 1990 to 1999 Lamborghini Diablo’s genuine replacement thermostat, which starts at around Php 17,000. 

Those are the symptoms of a bad or failing car thermostat, as well as the estimated cost of replacement. Again, trying to fix it yourself and learning as you go is admirable, but if you’re depending on your car for your livelihood, then better have it checked/replaced by a professional mechanic just to be sure.

For more useful and potentially helpful articles like this, keep it here on Philkotse.com.

Symptoms of a bad thermostat: FAQs 

1. How much do I have to pay to have my car’s thermostat replaced?

Answer: We cannot provide an estimate on a mechanic’s fee for replacing a car’s thermostat as it can vary depending on many factors. Typically, though, licensed professional mechanics working for a casa will cost a bit more but can be trusted.

2. What is a car thermostat?

Answer: A car’s thermostat is a device that regulates the flow of coolant in your car’s engine.

3. Is car coolant poisonous?

Answer: Yes, a car coolant is poisonous when ingested by humans. It also has adverse effects on soil, plants, and animals.

4. What do I do when my car overheats?

Answer: If your car overheats while you’re driving, pull over to a safe spot, shut off the car, and open the hood. Once it cooled down a bit, check your coolant levels.

5. How much is a car coolant temperature sensor? 

Answer: A car coolant temperature sensor’s typical price can range somewhere between Php 100 to Php 800 and above depending on the engine model, and the quality of the sensor itself.

Cesar G.B. Miguel

Cesar G.B. Miguel


Cesar Guiderone B. Miguel was born and raised in Iligan City, Lanao Del Norte. He graduated in 2010 with a Bachelor of Arts in English degree from Mindanao State University - Iligan Institute of Technology. He previously worked as a freelance writer for various websites, as a member of the Iligan City Disaster Risk Reduction Management's training staff, and as a medical sales representative.

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