How to test the car’s thermostat without removing it?

What is the role of a car thermostat?

The engine in your car operates in a temperature range of approximately 195-220 °F (91-104 °C). To help it operate in this range, your engine is helped by a thermostat.

In short, the thermostat reacts to changes in temperature, in this case, the coolant temperature, by opening or closing a valve to control the flow of coolant between the radiator and the engine. Thus, it is small, simple, and efficient.

The container of the expanding element in the thermostat faces and contacts the engine coolant. As the coolant temperature rises, the liquid in the container begins to expand, pushing the small rod that separates the center plate from the surrounding mounting base to open the valve.

After starting a cold engine, the coolant flows in what is called “the small circuit”. The passage to the main radiator is blocked by a valve, due to which the engine warm-up is accelerated.

When the set temperature threshold is reached, the valve begins to open, allowing some of the antifreeze to cool in the main radiator. When the engine warms up to operating temperature (over 90 °C or 191 °F), the valve opens completely and the entire antifreeze moves along in “the large circuit”.

The primary signs of a thermostat fault are reduced to two symptoms: constant “boiling” or the inability to warm up the engine, especially in winter.

There are many options for what happens if the thermostat is constantly open in a large or small circle. This will lead to overheating, antifreeze can be thrown out of the expansion tank, the normal pressure in the cooling system will change and not only.

Where is the car thermostat located?

The easiest way to know your thermostat location is to check the car user’s manual. In the section describing the cooling system, the location of the thermostat must be indicated.

At the same time, there is a fairly universal search method that is relevant for most cars.

Open the hood and find the thickest pipe, this will be the radiator hose that goes from the radiator to the engine. The second end of this pipe is connected to a thermostat. Get to the second end and determine the location of the thermostat housing and proceed with its removal.

Although I still advise you to first look into the user’s manual, and only after that search it using the method above.

How to check the car thermostat without removing it?

The easiest way to check if the thermostat is working is to diagnose it directly on the car. This requires the vehicle’s engine to be initially cold. The procedure itself is carried out according to the following steps:

Start the engine and let it work for 2-4 minutes. Touch the pipe leading to the top of the radiator with your hand. Normally, it should be cold, since at this stage the coolant circulates only through the heater and the engine. 

A hot pipe, however, indicates a thermostat fault. When checking, you need to pay attention to both upper and lower hoses. If they get hot, then this is also a bad sign.

There are situations when, after heating the coolant, the lower branch pipe remains cold. This tells us that the coolant cannot pass through the radiator. In both cases, we can talk about a thermostat malfunction, the needs to be removed and replaced with a new one.

There are situations when the valve gets stuck in an intermediate position. Here, it is neither closed nor open, so the engine warms up very slowly. Thus, you will have no choice but to remove the thermostat.

Causes of a bad or failing thermostat

1. The thermostat is stuck open

This malfunction is more common. Here, the thermostat valve opens completely at one point and remains in that state permanently. This happens because the force of the spring is not enough to return the valve to its original place. This is usually caused by corrosion, debris, or hardening of the liquid in the capsule.

Symptoms of a stuck open thermostat:

  • The engine takes much longer to reach the operating temperature, even in the warm season.
  • In cold weather, it does not fully warm up at all.
  • The heater either barely warms up or constantly blows cold air.
  • The engine power is reduced.
  • Fuel consumption increases.

The consequence of a stuck open car thermostat is that the engine life is reduced in the long run.

2. The thermostat is stuck closed

This is perhaps the most dangerous thermostat malfunction. When the coolant remains in the “small circuit”, engine overheating is inevitable. This happens for several reasons, and among them are corrosion and debris. 

Also, the capsule with liquid sometimes ceases to be airtight, and because of this, it cannot do its job. If you have this issue, it will not be possible to drive for long, and therefore it is very important to always remember how to identify it in a timely manner.

Symptoms of a stuck closed thermostat:

  • The arrow on the dashboard gauge goes off the scale.
  • If present, the corresponding warning errors will light up.
  • The motor can lose throttle response, stability, and it will even stall.
  • When the heater is running, you may feel the smell of antifreeze in the cabin.
  • White steam from under the hood can be present.

If the thermostat is stuck in the closed position, and you don’t take care of this problem fast, the engine will simply overheat and either stall or break. In general, it is a dangerous malfunction.

Premature opening or closing

In a working thermostat, the moment of opening and closing occurs at certain temperatures that are comfortable for the engine. If the valve is triggered ahead of time, then the coolant indicators are constantly outside the optimal range.

When the thermostat starts to open or close ahead of time:

  • The engine does not warm up to operating temperature as expected.
  • The heater never heats up to maximum.
  • Fuel consumption is slightly increased.

However, this is not a serious issue as when the thermostat is stuck open or closed, but it’s unpleasant. The reason for this malfunction is most often the natural wear of the thermostat. 

Late opening or closing

In this case, the engine regularly, although not critical, overheats, and you will feel the smell of burnt coolant in the cabin. The reasons for this fault are similar: normal wear and tear. 

Also, it is worth mentioning that using the wrong thermostat can cause this issue. This sometimes happens, since it is quite hard sometimes to find a suitable option for your car.

Incomplete opening or closing

An extremely rare fault. Dangerous only if the thermostat does not open completely. It is clear that sooner or later this will lead to engine overheating. If the valve does not close completely, then it will be almost the same as in the case of premature operation. 


Incorrect operation of the thermostat can cause a number of negative consequences, up to and including engine failure. Drivers find problems with this part in winter or summer periods. 

However, thermostat malfunctions have similar symptoms to other malfunctions, so to make a correct diagnosis you have to arm yourself with knowledge and spend a little time, or to take the car to an authorized repair shop.

Scroll to Top