How do I know if my car has a thermostat?

In most cars with a liquid-cooled engine, there is a thermostat that regulates the flow of coolant to keep the engine at its optimal temperature. You can usually confirm its presence by locating the thermostat housing near the engine, often connected to the upper radiator hose, or by checking your owner’s manual or service diagrams for your model.

What a thermostat does and where it’s usually found

The thermostat is a little temperature-controlled valve that opens and closes to regulate coolant flow. When the engine is cold, it stays closed to help the engine reach operating temperature faster. Once the coolant reaches a set temperature, the thermostat opens to allow coolant to circulate to the radiator and maintain steady heat. In most vehicles, you’ll find the thermostat inside a housing at the end of the upper radiator hose or inside the water pump/timing cover area, depending on the make and model.

Typical locations you might encounter

• Most conventional liquid-cooled engines have a thermostat housing at the end of the upper radiator hose, between the engine and radiator.


• Some designs place the thermostat inside the water pump housing or behind the timing cover.


• On certain engines, especially in front‑engine V‑shaped layouts, the housing is mounted on the front of the engine block near the hose connections.

Knowing where to look can help you confirm whether your car uses a thermostat and, if needed, where it’s likely to be located for inspection or replacement.

Signs that the thermostat may be faulty

Several symptom patterns can indicate a thermostat issue, though other cooling system problems can mimic them. Being aware of these signs can help you decide whether to test the thermostat or have a professional check it.

• Engine overheats quickly or runs hotter than normal, especially during driving that puts the engine under load.


• Engine warms up more slowly than usual and the heater takes longer to produce warm air.


• Temperature gauge or warning light behaves erratically, or stays at an unusually low or high reading.


• In some cases, you may notice poor heater performance or intermittently cold air from the HVAC system.


• There can be coolant leaks around the thermostat housing or gasket indicating wear or failure.

These symptoms do not prove a faulty thermostat by themselves, but they commonly point to a thermostat that is stuck open, stuck closed, or not opening at the correct temperature. A professional diagnosis can confirm the cause.

How to safely test or verify the thermostat

If you’re comfortable with basic car maintenance, you can perform a few checks to gauge whether the thermostat is working, while staying mindful of safety. If you’re unsure, err on the side of caution and consult a mechanic.

• Observe the upper radiator hose: as the engine reaches operating temperature and the thermostat opens, the hose should become noticeably hot, reflecting coolant flow to the radiator.


• DIY thermostat test (advanced): with the engine cool, locate the thermostat housing and remove the thermostat. In a pot of water heated to roughly 180–195 F (82–91 C), the thermostat should begin to open within that temperature range and show some movement. If it stays closed or opens far too early, it may be faulty and should be replaced with a gasket.


• Important safety notes: never open the cooling system while the engine is hot; always relieve pressure and use proper protective gear when handling coolant components.

If the DIY test or symptoms suggest a faulty thermostat, replacement is typically straightforward but requires correct part selection and a proper gasket to prevent leaks. When in doubt, a professional inspection can save time and avoid potential engine damage.

Summary

The thermostat is a common and critical part of most liquid-cooled car engines, responsible for maintaining optimal operating temperature. You can usually locate it in a thermostat housing connected to the upper radiator hose, though some designs place it elsewhere. Watch for symptoms like overheating, slow warm-up, or erratic temperature readings to gauge whether the thermostat may be at fault. For a definitive answer and safe handling, consult your vehicle’s manual or a qualified mechanic, especially if you’re not experienced with cooling system work.

How to test a thermostat without taking it out?

To test a thermostat without removing it, check for the system's basic function, like listening for the furnace to kick on when you change the temperature setting. For a car, monitor the dashboard temperature gauge while the engine warms up, or use a non-contact infrared thermometer to measure the temperature of the thermostat housing to see when it gets hot. 
For a home thermostat

• Test for clicks: Turn the thermostat from the "off" position to a higher heat setting. You should hear a clicking sound from both the thermostat and the furnace, indicating the signal is being sent. 
• Observe the system: After turning the thermostat to cool, check if the air coming from the vents is actually cooling. If it's not cooling or the temperature is inconsistent, there could be a problem. 
• Use a smartphone app: If it's a smart thermostat, use the manufacturer's app to run a system test, which may check for basic functionality. 

This video demonstrates how to test a home thermostat using a multimeter: 56sJersey Mike HVACYouTube · Mar 3, 2024
For a car thermostat

• Monitor the temperature gauge: Start the engine and let it idle. Watch the dashboard temperature gauge. It should rise steadily to the normal operating temperature and stay there. If it stays cold for an unusually long time or climbs to the red zone, the thermostat may be stuck open or closed, respectively. 
• Feel the radiator hose: As the engine warms up, carefully feel the upper radiator hose. Initially, it should be cool, and then it should start to get hot as the engine reaches its operating temperature. If it remains cold when the engine is hot, the thermostat may be stuck closed. 
• Use an infrared thermometer: For a more precise temperature reading, use an infrared thermometer to measure the temperature of the thermostat housing as the engine runs. You should observe a jump in temperature when the thermostat opens. 

You can watch this video to learn how to test a car thermostat without removing it: 57sYou WrenchYouTube · Mar 29, 2021

What are the symptoms of a bad thermostat in a car?

Symptoms of a bad car thermostat include erratic or fluctuating engine temperature gauge readings, engine overheating or taking too long to warm up, and poor cabin heater performance. Other signs can be poor fuel efficiency, coolant leaks, and strange noises from the engine.
 
Common symptoms

• Erratic temperature changes: The temperature gauge on your dashboard may swing up and down unexpectedly, or stay in the red zone. 
• Engine overheating: If the thermostat is stuck closed, it prevents coolant from reaching the radiator, causing the engine to overheat and potentially causing severe damage. 
• Engine running too cool: If the thermostat is stuck open, the engine will take a very long time to warm up or may never reach its proper operating temperature. 
• Poor cabin heater performance: When the thermostat is stuck open, there may be little to no heat in the cabin because the coolant is circulating too much and never gets hot enough. 
• Leaking coolant: A stuck thermostat can cause pressure to build up in the cooling system, leading to leaks from the thermostat housing or hoses. 
• Poor fuel economy: A thermostat stuck open can cause the engine to run too rich, leading to decreased fuel efficiency. 
• Strange sounds: You may hear gurgling, boiling, or knocking sounds from the engine. 
• Check engine light: A failing thermostat, especially one stuck open, can trigger a check engine light, sometimes with a specific code like P0128. 

How much does it cost to replace a thermostat?

The average cost to replace a thermostat is between $100 and $350, but this can vary depending on the type of thermostat, labor costs, and any additional complications. A basic manual thermostat may cost around $100-$150 total, while a smart thermostat can increase the total price to $350 or more. Professional installation costs can range from about $100 to $200 in labor, depending on the complexity of the job and the hourly rate of the professional, which can be between $50 and $150 per hour. 
Factors that influence the cost

• Type of thermostat: Basic manual thermostats are the least expensive, while smart and programmable thermostats cost more due to their advanced features. 
• Labor: The cost of labor depends on the complexity of the installation and the professional's hourly rate. Some professionals may charge a flat fee for installation. 
• Location: The cost of professional installation can vary depending on your geographic location. 
• System compatibility: If your existing system requires a different type of thermostat or has unique wiring, the installation cost will likely increase. 
• Additional work: If the thermostat needs to be moved to a new location or requires new wiring, the cost will be higher. 

DIY vs. professional installation

• DIY: If you are comfortable with basic electrical work, you may be able to replace a thermostat yourself to save on labor costs. However, improper installation can lead to damage to the thermostat or your HVAC system, or pose a risk of electric shock. 
• Professional: Hiring a professional ensures the job is done correctly and can provide peace of mind. Many companies offer installation services that may include a labor warranty. 

How to get an accurate estimate
To get a precise cost for your situation, it's recommended to: 

• Determine the type of thermostat you need.
• Get quotes from several local professionals, such as HVAC technicians or electricians.
• Inquire about their hourly rates or flat-fee installation costs.

How do you know if your car has a thermostat?

Your vehicle's thermostat is a vital tool that is designed for measuring the temperature of the coolant in the engine. You can usually find the corresponding thermostat gauge, or dial, on the dashboard.