5 Symptoms of a Bad Thermostat (and Replacement Cost)
Is your engine overheating or taking forever to warm up? The root cause may be a faulty thermostat improperly regulating coolant flow.
Learn to recognize the symptoms of a stuck open or stuck closed thermostat, a small but extremely important component for proper engine temperature. Fortunately, the cost to replace the thermostat isn’t sky high and DIY replacement is easier than you think in some cases.
Bad Thermostat Symptoms
A failing car thermostat can significantly impair your engine’s performance and functionality. Fortunately, there are several easily recognizable symptoms that indicate a faulty thermostat.
It’s important to make replacing failed thermostat a high priority in order to prevent irreversible engine damage that can be caused by prolonged overheating.
See Also: Symptoms of a Bad MAP Sensor
#1 – Temperature Gauge Reading Higher (or Lower) Than Normal
When starting your car after it has been sitting overnight or for a few hours, the temperature gauge needle should be on the “Cold” side. As you drive, the needle should slowly move up to about the midway point, indicating the optimal engine operating temperature.
If the thermostat is stuck closed, it will prevent coolant from flowing into the engine, causing the temperature gauge to continue rising until it reaches the “Hot” end. You may even notice steam coming from your engine compartment. Continuing to drive in this condition increases the risk of engine damage.
Always monitor the engine temperature gauge and pull over to allow the engine to cool down if you notice the temperature rising higher than normal.
Conversely, if the thermostat is stuck open, the temperature needle will move up much slower than usual and may stop before reaching its normal mid-point. If you notice this, try turning on your heater. If it doesn’t blow warm air, it’s likely a broken thermostat.
Related: Is Your Temperature Gauge Not Moving? (Common Causes)
#2 – Sudden Air Temperature Changes Inside the Vehicle
One of the more noticeable symptoms of a faulty thermostat is sudden changes in the air temperature inside your vehicle. If you experience a sudden drop in temperature followed by a rapid spike, it’s a strong indication that your thermostat is not regulating the coolant flow properly.
These temperature fluctuations can be uncomfortable for passengers and are often unrelated to your HVAC settings. If you notice inconsistent air temperatures that don’t match your climate control settings, get your thermostat checked out.
#3 – Coolant Leaking
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.
A stuck-closed thermostat prevents coolant from circulating through the engine, causing a buildup of pressure in the cooling system. As a result, coolant may leak from the thermostat housing, radiator, or hoses.
Coolant leaks can be identified by puddles of green, pink, or orange liquid under your vehicle. If you suspect a coolant leak, get it checked out asap, since low coolant levels can lead to engine overheating and costly damage.
#4 – Rumbling Noises
Specific, unusual noises coming from your vehicle’s engine or radiator are another sign of a thermostat problem. These sounds may resemble rumbling, knocking, boiling, or gurgling.
Rumbling noises often occur when coolant is unable to circulate properly due to a stuck-closed thermostat, causing the engine to overheat. Boiling or gurgling sounds may indicate that your coolant is overheating and bubbling inside the radiator or engine.
#5 – Heater Malfunction
In colder climates or during winter months, a stuck-open thermostat can cause issues with your vehicle’s heating system. A thermostat that remains open, allows coolant to flow continuously through the engine, even when it’s not needed.
This may not allow the engine to reach its optimal operating temperature, resulting in cool air blowing from the vents when you have the heater turned on.
If you find that your heater is not working effectively, particularly if the air remains cool regardless of the temperature setting, it could be a sign of a stuck-open thermostat in addition to typical heater core related issues.
Causes of Thermostat Failure
- Age and Wear – A car thermostat won’t last forever. Constant exposure to temperature changes and repetitive motion can cause the thermostat to wear out over time.
- Coolant Contamination – Dirty or acidic coolant can clog or corrode the thermostat, causing it to stick or malfunction.
- Overheating – Frequent engine overheating puts additional stress on the thermostat, potentially causing warping or premature failure.
- Incorrect Installation – Installing the thermostat upside down or overtightening the housing can lead to improper functioning or damage.
- Manufacturing Defects – In some cases (usually with cheap aftermarket brands), a faulty thermostat may be caused by poor construction or subpar materials. Sticking with OEM parts reduces this risk.
- Impact Damage – Depending on where the thermostat is located, physical damage to the thermostat housing or surrounding components from road debris or accidents can cause thermostat failure.
Car Thermostat Replacement Cost
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The good news for those with a faulty thermostat is that replacement is relatively affordable when compared to other car repairs.
However, the exact price will vary depending on your vehicle’s make and model, OEM vs aftermarket parts, your area (it’s generally more expensive the closer you are to a city center), and where you take your vehicle to have the repair done.
On average, you can expect to pay between $140 and $300 for a professional mechanic to replace your car’s thermostat. But it can be more in some cases.
The thermostat itself typically costs between $20 and $80, but this price can be higher for luxury or high-performance vehicles. Labor costs will make up the bulk of the total expense, ranging from $100 to $300 or more, especially if the work is done at a dealership.
Most mechanics charge between $100 and $150 per hour for their services these days, and a thermostat replacement job usually takes 1 to 2 hours to complete. As a result, labor costs will almost always exceed the price of the thermostat itself.
While some car owners attempt to save money by replacing the thermostat themselves, it’s generally recommended to have a professional handle the job unless you have significant experience working on cars or your vehicle’s thermostat is easily accessible. In addition to ensuring proper installation, a professional mechanic can diagnose any other underlying issues that may be affecting your vehicle’s cooling system.
When considering thermostat replacement, it’s best to get multiple quotes from reputable mechanics and service centers to make sure you’re getting a fair price.
And while it should go without saying, if your vehicle is still under warranty, the cost of the thermostat replacement may be fully or partially covered, so be sure to check your warranty terms before proceeding with any repairs.
How a Car Thermostat Works
The main function of a car thermostat is to regulate the flow of coolant to maintain optimal engine temperature.
When you start your car after it has been sitting for a while, the thermostat remains closed, allowing the engine to warm up quickly by preventing coolant from circulating through the radiator.
As you continue to drive, the engine temperature rises, causing the thermostat to open once it reaches its predetermined temperature (usually around 195°F to 220°F). This allows coolant to flow through the radiator, where it is cooled before returning to the engine to absorb more heat.
The thermostat constantly adjusts its open/closed position based on the engine’s temperature, maintaining a balance between the hot coolant coming from the engine and the cooled coolant returning from the radiator. This process helps keep the engine operating at its optimal temperature range, typically around the midpoint of the temperature gauge.
If you leave your car idling for an extended period, the thermostat may remain closed, as it takes longer for the engine to reach its operating temperature when not under load. However, once you start driving and the engine works harder, the thermostat will open to prevent overheating.
The thermostat’s operation is controlled by a temperature-sensitive element, such as a wax pellet or a bimetallic strip, which expands or contracts in response to changes in coolant temperature. As the element expands, it opens a valve within the thermostat, allowing coolant to flow through. When the coolant temperature drops, the element contracts, closing the valve and restricting coolant flow.
In addition to the thermostat itself, a coolant temperature sensor provides input to the vehicle’s engine control module (ECM), which monitors and regulates various engine functions. This sensor helps the ECM determine when to adjust the fuel injection, ignition timing, and other parameters to maintain optimal performance and fuel efficiency.
Environmental factors, such as outside air temperature, can also influence the thermostat’s operation. However, a properly functioning thermostat should be able to adapt to these conditions and maintain the engine’s operating temperature within the desired range.
Preventive Maintenance
Performing regular preventive maintenance of your vehicle’s cooling system is more important than most car owners think. Here are a few things you can do to extend the life of your thermostat and avoid costly engine repairs due to overheating.
Regular Coolant Flushes
Over time, coolant can become contaminated with rust, debris, and other particles that can clog or damage the thermostat. Flushing the cooling system and replacing the coolant at the intervals recommended by your vehicle’s manufacturer helps maintain the health of your thermostat and other cooling system components.
Consult your owner’s manual or a trusted mechanic to determine the appropriate schedule for your vehicle. The cost of a coolant flush is only slightly more than what many people spend on an oil change. A DIY radiator flush is also very doable for many individuals who want to save some money.
Thermostat Inspection
Whenever you have your coolant flushed professionally, ask your mechanic to inspect the thermostat for any signs of wear, damage, or malfunction. A good repair shop will often automatically do this.
They can check for leaks, corrosion, or sticking, and recommend replacement if necessary. Catching thermostat issues early can prevent more serious and expensive problems down the road.
Fix Coolant Leaks ASAP
If you notice any coolant leaks, have them repaired as soon as possible. Low coolant levels can cause the engine to overheat, putting additional stress on the thermostat and other cooling system components.
Regularly inspect your vehicle for signs of coolant leaks. While a small puddle is easily noticeable, sometimes coolant loss isn’t so obvious.
Monitor Engine Temperature
Pay attention to your vehicle’s temperature gauge while driving. If you notice the gauge consistently reading higher or lower than normal, or if the engine overheats frequently, have your cooling system inspected by a professional.
There’s no better person to catch a thermostat issue early on than the person who regularly drives the car. If you get into the habit of glancing at the temp gauge during your drives, a small change will often be apparent.
Use the Right Coolant
Always use the type of coolant specified by your vehicle’s manufacturer. Don’t just simply buy a yellow jug of Prestone because that’s what your dad taught you 20+ years ago. As vehicles have become more sophisticated, different manufacturers call for different types of coolant.
Using the wrong coolant or mixing different types can cause corrosion and damage to the thermostat and other cooling system parts. If you’re unsure about which coolant to use, consult your owner’s manual.
Keeping the Radiator Clean
A dirty or clogged radiator can reduce its ability to efficiently cool the engine, putting additional strain on the thermostat. Periodically clean any debris from the front of the radiator, such as leaves or insects, to make sure it gets proper airflow.
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Had a friend replace the radiator and thermostat in a 2014 Dodge Durango. A couple months after the gauge kept going up and back down after a couple of seconds. Thought it needed to be bled. Tried bleeding. Thought all air was out so turn off car to take the funnel out and replace cap when all of a sudden all of the coolant comes bubbling out. The car was off!! Any idea as to what is going on?
Sounds like the cooling system on the vehicle needs to be bled correctly. As the engine gets warmer, the coolant level will rise. Bubbles in the engine block may only be able to escape once the thermostat has opened, allowing the coolant from the block to circulate back to the radiator.
What is a reasonable cost for replacement of thermostat on a 2013 better turbo 2.0? Dealership vs. non-dealership?
2013 VW Beetle Turbo 2.0
1997 Nissan Pathfinder the temp gauge only ever goes up to about half way but the engine definitely feels hotter than that when ur close to it.
The halfway point is what’s called “normal operating temperature”. This is the temperature your engine tries to maintain while it runs, and is completely normal.
You don’t want your temperature gauge to creep up past this point. You’ll only see this happen when your vehicle is overheating.
Hi I purchased a thermostat for my 318i e46 how do I open it per say…. Temperature test?
Some people put them in a pot of boiling water with a thermometer to see if they open and close as expected.
Temperature gauge I’m my vehicle is going up about 3/4 & would over heat or go back down a little bit if I keep it running. Why is this happening?
There are many things that could cause your vehicle to overheat. The thermostat is one of them. Check your coolant level to make sure you have enough coolant, and bleed the cooling system to make sure you don’t have any air pockets trapped anywhere.
Another sign of a faulty thermostat is: After running the vehicle to have normal operating temperature, grab onto the lower hose. If it is cold, thermostat is faulty.
Yes, this is one way to test it. However, you often have to wait for a bit even after your vehicle hits its normal operating temperature. Be careful trying this because radiator hoses will get hot enough to burn you.
When you start a cold vehicle, the thermostat should be closed. After your vehicle hits operating temperature, the engine temperature has to increase a bit more for the thermostat to open (since the goal of the thermostat is to keep the engine right around this operating temperature). Next, the hot coolant from the engine has to heat up all the coolant in the radiator before you start to notice a temperature change in the lower radiator hose.
This test probably won’t work reliably for a thermostat that is stuck open. A thermostat that is stuck open is still faulty, but the lower radiator hose will still warm up over time.
Your information has been so helpful,
Thank you very much
Glad you liked it!
Thanks very much i am having the exact issue with my Car and thought its the Radiator, but after Mechanic checking the car he found out it is Themosart