Can you fix a bad thermostat in a car?

Yes. In most vehicles, a faulty thermostat can be fixed by replacing the thermostat assembly and gasket; this often resolves overheating or heater problems and returns the engine to normal operating temperature. However, similar symptoms can be caused by other cooling-system issues, so a proper diagnosis is essential.

In this article, we explain how thermostats regulate engine temperature, common failure modes, how to diagnose a faulty unit, steps for a DIY replacement, safety considerations, and when to call a professional.

How a car thermostat works and why it fails

The thermostat sits in the coolant path and acts as a valve that opens and closes based on temperature. When the engine is cold, it stays closed to help the engine warm up quickly. Once the coolant reaches the operating temperature, the thermostat opens to allow coolant to flow to the radiator. Over time, thermostats can stick open or closed due to age, deposits, improper coolant mixtures, or gasket leaks.

Common failure modes

Sticking open can cause the engine to run cooler than normal and the heater may feel weak. Sticking closed can trigger rapid overheating and potential engine damage. Sometimes a thermostat fails intermittently, or the seal leaks coolant.

Recognizing a bad thermostat: symptoms to watch

Check for patterns in the engine cooling and heating system; these signs often point to a thermostat issue rather than a random leak or water pump problem.

• Engine runs cooler than expected after startup, with the gauge staying near the bottom and the heater not getting warm.


• Engine overheats quickly, especially on hot days or under heavy load, or the cooling fan runs constantly.


• Temperature gauge fluctuates or never seems to reach the normal operating range.


• Heater produces little or no heat even when the engine is fully warmed.


• Coolant leaks around the thermostat housing or smell of coolant near the engine bay.

Diagnosing thermostat issues should consider other cooling-system components, such as the radiator cap, water pump, hoses, and coolant quality.

DIY replacement: what you need to know

If you have basic mechanical skills and proper tools, replacing a thermostat is a common maintenance job. Prepare for working around hot coolant and to properly bleed air from the system after refilling.

1. Get the right parts: a new thermostat (make/model specific) and a new gasket or o-ring, plus fresh coolant compatible with your vehicle.


2. Safety first: ensure the engine is cold, disconnect the battery, and wear eye protection and gloves. Jack up the car if needed and work on a stable surface.


3. Drain or partially drain coolant as needed to access the thermostat housing, typically located where the upper radiator hose connects to the engine.


4. Remove the thermostat housing, take note of orientation, and remove the old thermostat and gasket.


5. Clean the mating surfaces, install the new thermostat in the correct orientation (spring toward the engine), and reseal with the new gasket or o-ring. Reinstall the housing and tighten to spec.


6. Reconnect hoses and refill the cooling system with the correct coolant mixture. Bleed air from the system if your vehicle requires it and run the engine with the radiator cap off until the thermostat opens and the air bubbles stop.


7. Check for leaks, top off coolant as needed, and run the engine to normal operating temperature to ensure proper function. Dispose of old coolant properly.

Note that some vehicles require specific bleeding procedures or have electronic thermostats that are controlled by the engine ECU; in those cases, refer to the service manual for exact steps.

When to seek professional help and typical costs

If you are unsure about the diagnosis, do not have the right tools, or the cooling system has other issues beyond a faulty thermostat, seek a professional mechanic. A thermostat replacement is usually one of the more affordable cooling-system repairs, but costs vary by make, model, and whether other components need attention.

Typical costs for a thermostat replacement range from roughly 150 to 350, including parts and labor for many passenger cars. More complex engines, luxury brands, or jobs that require additional cooling-system work (such as replacing a faulty water pump or radiator) can raise the price.

What else to check if symptoms persist

If replacing the thermostat does not fix the issue, consider these possibilities and steps to diagnose further:

• Coolant flush or replacement if coolant is old or contaminated.


• Inspect and replace the radiator cap if it’s not maintaining system pressure.


• Test or replace the water pump if the impeller is worn or leaking.


• Rule out air pockets and bleed the cooling system correctly after any work.


• Check for external leaks and inspect hoses for cracks or collapse.

Persistent cooling issues can also indicate more serious problems such as a blown head gasket or a cracked cylinder head, which require professional assessment.

Summary

A faulty thermostat is a common cause of cooling and heating problems in cars, and many repairs involve replacing the thermostat and gasket, along with a careful bleed and refill of coolant. Proper diagnosis is essential to avoid unnecessary work, and safety should be the priority when dealing with hot coolant and pressurized systems. If in doubt, consult a qualified mechanic.

What happens when your thermostat in your car goes out?

Common symptoms of a bad car thermostat include an overheating engine, a temperature gauge that fluctuates or reads too high/low, coolant leaks, and poor heating performance in the cabin. Other signs include strange noises like rumbling, steam from under the hood, or the "check engine" light turning on. A thermostat stuck open can cause the engine to run cold and take a long time to warm up, while one stuck closed can lead to rapid overheating.
 
Symptoms of a bad thermostat

• Overheating engine: This is a major symptom, often caused by a thermostat stuck in the closed position, preventing coolant from flowing to the radiator. 
• Erratic temperature gauge: The gauge may swing between hot and cold, or stay at the maximum or minimum, indicating the thermostat is not regulating temperature correctly. 
• Poor heating performance: If the thermostat is stuck open, the engine may not reach its proper operating temperature, leading to cold air from the vents, especially in cold weather. 
• Coolant leaks: A leaking gasket or housing around the thermostat can be a sign of a bad thermostat, especially when caused by the excessive heat and pressure from an overheating engine. 
• Strange engine noises: Boiling, gurgling, or knocking sounds can indicate that the coolant is boiling due to overheating. 
• Steam from under the hood: This is another sign of an overheating engine caused by a stuck-closed thermostat, leading to high pressure in the cooling system. 
• "Check Engine" light: In some cases, a faulty thermostat can trigger the check engine light, sometimes with a specific code like P0128. 
• Engine runs cold: If the thermostat is stuck open, the engine may take an unusually long time to warm up and will not reach its optimal operating temperature. 

How do I tell if my thermostat is bad in my car?

You can tell if your car's thermostat is bad by observing symptoms like engine overheating or under-heating, erratic temperature gauge readings, slow cabin heater warmup, and poor fuel efficiency. Another sign is a leaking coolant or strange engine noises. You can also use an OBD-II scanner to check for a specific error code like P0128, which indicates the engine is taking too long to warm up.
 
This video demonstrates how to identify symptoms of a failing thermostat: 59sEasyAutoFixYouTube · Nov 11, 2023
Signs of a failing thermostat

• Engine overheating or under-heating: If the engine gets too hot, the thermostat may be stuck closed, preventing coolant from reaching the radiator. If the engine runs cold, the thermostat may be stuck open, and the engine never reaches its optimal operating temperature. 
• Fluctuating temperature gauge: A faulty thermostat can cause the temperature gauge to move erratically, spiking into the hot zone and then dropping back down unexpectedly. 
• Poor cabin heating: A slow-to-warm cabin or consistently weak heat can be a sign that the thermostat is stuck open and the coolant isn't getting hot enough. 
• Poor fuel efficiency: An engine that constantly runs too cool due to a stuck-open thermostat can lead to decreased fuel economy. 
• Check engine light: The "check engine" light may illuminate, especially if there is a specific code like P0128, which suggests the engine is taking too long to reach its operating temperature. 
• Unusual engine noises: Gurgling or rumbling sounds from the engine can be a sign of a thermostat problem. 
• Coolant leaks: While the thermostat itself doesn't leak coolant, the pressure caused by overheating can lead to leaks from other components like hoses or the radiator. 

You can watch this video to learn how to test the thermostat in your car: 49sTimeless Steel GarageYouTube · Dec 29, 2024
What to do

• If you experience any of these symptoms, it is best to have your vehicle checked by a mechanic to ensure a proper diagnosis. 
• A bad thermostat can cause severe engine damage, so addressing the issue promptly is important to prevent costly repairs. 

How much does it cost to replace a thermostat in a car?

The average cost to replace a car thermostat ranges from $150 to $500, though luxury or older vehicles may cost more. The price varies based on the vehicle's make and model, with the thermostat part itself costing between $20 and $80, and labor making up the rest of the cost. For some high-end cars, replacement can exceed $1,000.
 
You can watch this video to learn about the factors that influence the cost of thermostat replacement: 59sThe Auto ShopYouTube · Feb 1, 2019
Cost breakdown

• Parts: A standard thermostat costs approximately $20 to $80. 
• Labor: Labor costs can range from $120 to $220, or more depending on the vehicle. 
• Total: The total average cost is between $150 and $500, but this can be higher or lower depending on the vehicle's specifics. 

Factors that affect the cost

• Vehicle make and model: Luxury and high-performance cars often have more expensive parts and more complex systems, which increases the overall cost. 
• Location: Labor costs can vary significantly by region. 
• Labor time: The job can take up to two hours, which factors into the final price. 
• Type of replacement: Replacing a thermostat is often recommended over repairing it to ensure a lasting solution, but this means purchasing a new part. 

This video explains why it can be challenging to replace a thermostat in some cars: 58sMechanic BaseYouTube · Mar 14, 2023
Can you drive with a bad thermostat?
It is not recommended to drive with a bad thermostat, as it can cause the engine to overheat and potentially lead to more significant, expensive damage.

How to fix a bad thermostat in a car?

Locate the thermostat housing (usually near the engine block), drain some coolant, remove the housing, and carefully replace the thermostat. Ensure you follow the manufacturer's guidelines and use the correct thermostat for your vehicle.