The thermostat is one of the more important components of an automobile engine. Think of the cooling system as two parts; the engine assembly and the radiator, with coolant circulating between the two.
The thermostat is a valve between the two that closes to allow the engine to heat up to running temperature, and then opens to allow the coolant to circulate. The engine assembly generates heat, and the coolant carries that heat to the radiator, where it is absorbed.
Engines run best at a certain temperature; when they are cold, they run very inefficiently, and if they get too hot, problems arise. Various sensors and heat management strategies are employed, but at the core is the thermostat. It holds coolant in the engine until operating temperature is achieved; then, it opens and allows coolant to circulate to the radiator and maintain that temperature.
Signs that a thermostat isn’t working properly can be that an engine runs too hot, too cold, or won’t maintain a consistent temperature. If a vehicle takes too long to warm up, that can indicate that the thermostat isn’t sealing up when closed. If there is no coolant temperature gauge, this can also show as a heater that doesn’t get hot. On many modern vehicles, a trouble code will be set, and the check engine light will turn on if the coolant temperatures indicate a thermostat problem. P0128 is the most common thermostat-related engine code.
Usually the thermostat is located where the upper radiator hose meets the engine assembly, and most of the time (though not always), it is fairly accessible.
What’s the Price of a Car Thermostat?
On average, for most vehicles, it costs about $170-$250 to replace a thermostat by itself. It can be less if combined with other services.
For some specific examples of the thermostat replacement cost on some common vehicles, we are using $150 an hour as a labor rate:
| Car Model | Labor Time & Cost | Coolant Cost | Aftermarket Thermostat Cost (Excl. Labor) | Factory Thermostat Cost (Excl. Labor) |
|---|---|---|---|---|
| 2010 Subaru Impreza (2.5L engine) | $105 (0.7 hrs) | $20 | Gates part for $23 | $36 |
| 2003 Chevrolet S10 Blazer (4.3L engine) | $165 (1.1 hrs) | $13 for a gallon | Stant part for $18 | $49 |
| 2012 Toyota Tundra (5.7L engine) | $210 (1.4 hrs) | $18 | Aisin part for $68 | - |
| 2000 Volkswagen Passat (2.8L engine) | $735 (4.9 hrs) - the thermostat is behind the timing cover, requires timing belt removal | $15 | Mahle part for $21 | $58 |
As far as cost savings go, the most economical time to replace a thermostat is before it fails. There is no regular scheduled replacement interval for the part, and they don’t normally wear out from use.
However, if an engine or cooling system is being serviced for other reasons, then installing a new thermostat during related work can be good preventative and inexpensive maintenance.
A general rule is to replace the thermostat if the vehicle has overheated (regardless of cause), and to replace the thermostat if it is in any way suspected of having a problem.
Other Things That May Come Up
Given that replacing the thermostat involves draining about a gallon of coolant, the most common service that goes along with thermostat replacement is a cooling system flush. With the system open and partially drained, it is fairly easy to complete. If the cooling system in any way indicates a need for a general scheduled service, it will probably be recommended.
Related Article: Coolant Flush Service Cost Guide
Many modern cooling systems are complex internally and prone to trapping air. In many cases, normal service procedures require the system to be purged of air with a vacuum device after draining.
If a shop lacks the equipment, it can take a long time to work the air out of a cooling system, and sometimes components (such as the heater) won’t work normally for a while. In the worst cases, a car may continue to run hot until it is fully purged.
An overheating event can easily damage or render the thermostat unreliable. This means that it’s not always easy to distinguish cause from effect.
Sometimes, a thermostat can be diagnosed and replaced, and once it is working, another underlying cause for engine overheating is found. After replacement, attention should be paid to the engine temperature and other components to verify that everything is working as it should.
How Thermostats Fail
A thermostat is basically a valve operated by a rod set in a cylinder filled with wax. When the temperature of the cylinder reaches a certain point (180 to 190 degrees typically), the wax melts and expands; pushing the rod up and opening the valve.Mechanical failures are possible as deposits sometimes build up and prevent the valve from closing fully. The rod can also migrate from a centered position with wear and fail to fully open or close, or it can stick in one position. Sometimes the valve has a rubberized coating, which can be damaged by the wrong coolant or contaminants in the coolant.
Another type of failure that can result from overheating is if the wax expands and over-pressurizes the cylinder it resides in due to excessive temperatures, and the liquid wax then leaks past its seals. If even a small amount of wax is lost, the valve won’t open fully or at all.
It is fairly easy to test a thermostat by placing it in water that is being brought to a boil. By measuring the temperature of the water at the point where the valve opens and comparing that to the desired engine operating temperature, the valve can be verified as operational or not.
In practice, this is very seldom done. Most thermostats are much cheaper than the labor time involved in bench testing them. The normal recommendation, if one is suspect, is to replace it almost as a maintenance item.
FAQ
It can last the life of the vehicle, but can also be damaged by overheating, corrosion, or contamination in the cooling system.
Being an inexpensive part with a critical function, the thermostat is typically replaced whenever it is suspected of having a problem. It’s also commonly replaced when doing other cooling system repairs.
It can be done, but it’s a bad idea. The engine will typically run well below its optimal temperature, which can foul spark plugs, cause carbon build-up, and clog the catalytic converters.
If the engine runs too hot or too cold or takes too long to warm up, those are possible signs. Many newer vehicles will also set an engine light if sensors indicate a thermostat is not operating properly.
On most vehicles, probably. Take care to install the part correctly, replace the seal or o-ring, use the correct coolant, and be patient in refilling the system. Watch the coolant gauge to make sure the running temperature does what it should afterwards and there’s no air trapped in the system.