Problems with the car thermostat? Possible causes and solutions

Does the engine warm-up too quickly or does it not reach a temperature at all? Then the thermostat could be faulty. In addition to the typical symptoms of a faulty thermostat, we inform you here about the function of the component, the causes and consequences of a defect and the costs of a possible replacement of the faulty thermostat.

What does a thermostat do and how does it work?

The task of the thermostat is to regulate the temperature of the cooling water: It should quickly bring the engine to the optimum operating temperature and keep it constant. For this purpose, the thermostat has a double cooling water circuit, one small and one larger. 

If the engine is started from cold, the thermostat is initially closed and the cooling water flows only in the smallest circuit (water only flows through the engine). This allows the engine to reach operating temperature more quickly, while the coolant remains in the radiator for the time being. 

Among the essential components of the small cooling circuit are the water pump, the heat exchanger for heating inside and the engine block itself. When the engine operating temperature is reached, the thermostat opens and the radiator circuit connects to the small cooling circuit: This is called the large cooling circuit. 

To regulate the flow of the cooling water and therefore the temperature of the engine, there are essentially two types of thermostats: the classic thermostat, which controls the flow of the cooling water by means of an expansion element, and the controlled thermostat from a map, which has an electric actuator. 

What both types of thermostats have in common is that they have a spring plate. These plates open or close the path between the small and large cooling circuit. When cold, the spring plate closes the path to the cooling water cooler. 

When the cooling water reaches the thermostat opening temperature, the expansion element or motor control unit opens the plate via the electric actuator and thus opens the way for the cooling water. This closing and opening occur every time the temperature drops below or reaches the opening temperature. 

Call the specialist towing service within your vicinity when your vehicle is overheating and needs repair.

What are the symptoms of a bad thermostat?

Basically, there are only two clear signs of a bad thermostat. Either the engine reaches operating temperature too quickly, or it does not reach it at all (or very slowly, during a long journey for example). 

In both cases the thermostat is blocked: In one case the thermostat is permanently closed and in the other, the thermostat is permanently open. These problems can occur with both the classic thermostat and the map-controlled thermostat.

To be able to detect any of these cases, the instrument cluster must have a cooling water temperature gauge. If the display is missing, the defect is often noticed too late. In the event that the cooling water becomes too hot, each cooling system is equipped with a pressure relief valve through which the pressure is released in the form of water vapor. 

This safety valve is usually found in the expansion tank cover or in the radiator cover. At the latest when the water vapor is released, the red coolant light in the instrument cluster should start flashing, causing the vehicle to stop. In rare cases, the radiator fan can compensate for a blocked and closed thermostat.

The defect can be recognized by the fact that the noise of the fan running constantly is heard. Equally rare is the fact that a stuck open thermostat is noticed by the indicator light. In the long run, the warning light could come on due to malfunctions in the lambda probe and/or catalyst as they cannot function normally at low temperatures.

What can be the cause of a thermostat defect? 

The most common cause of a permanently open thermostat is a stuck piston. Corrosion also damages the thermostat and can lead to a defect. Occasionally a foreign object also jams the open thermostat so that it can no longer close when it cools down. 

If the spring plate of the thermostat is supported not only by the thermostat itself but also by the thermostat housing, it often happens that the supports on the housing break, so that the workpiece will lock only by loosening the flow of cooling water.

Over time, the gasket inside the expansion element also leaks, allowing the wax, which is the actual expansion material, to escape. As a result, the element no longer has enough wax to expand and the thermostat remains closed.

For map-controlled thermostats, the most common cause is a faulty electric actuator. Over time, cooling water may enter the actuator, paralyzing it. Is it possible to continue driving with a faulty thermostat?

Case 1: The thermostat does not open

The worst case of a thermostat that no longer opens is engine damage due to overheating. Older engines had small cast iron blocks and aluminum heads. They were unable to withstand overheating and the cylinder head gaskets broke quickly due to overheating.

Modern engines, in which both the engine block and cylinder head are made of an aluminum alloy, can withstand the increased heat much better. 

A bursting cooling water pipe or rupture of the plastic cooling water flange can lead to abrupt draining of the cooling system and can also cause the piston to seize due to overheating. Cooling water slowly drains through the pressure relief valve or a rubber seal slowly fails and the cooling water gradually drips. Nowadays this scenario is becoming rarer and rarer.

Case 2: The thermostat does not close

At first, a permanently open thermostat appears less dramatic. The consequences, however, are much more insidious. If the engine is not hot as it should, all mechanical components are subject to considerable wear, as all mounting dimensions are designed for the operating temperature, they do not expand as they should. In addition, the engine always runs in the warm-up phase, which means that it is permanently too rich and therefore the fuel consumption is significantly higher.

In addition, the vehicle may no longer meet the exhaust emissions standard because the catalyst or particulate filter is no longer able to withstand the exhaust gases of a rich mixture and the conversion rate can no longer be achieved or worse it is clogged.

What to do with a faulty thermostat?

If your instrument cluster has a cooling water temperature gauge, you can try driving to the nearest garage with a faulty or no longer opening thermostat. You should always keep an eye on the temperature so that the engine does not overheat too much. 

The following behavior can help:

  • Park the vehicle outside and let the engine cool down (faster with the hood open).
  • Set the interior fan to “ECO” or turn off the air conditioning, set the interior temperature and the fan to the maximum to cool the engine.
  • Drive at night as the outside temperatures will be lower.
  • Remove the thermostat. 

If the thermostat no longer closes, you can easily complete the journey and then take your vehicle to the nearest garage. You should avoid driving only with a full load. Driving under full load can further heat the engine and at the same time the wear on the engine increases.


How much does it cost to change a thermostat?

Thermostats are usually not very expensive, usually, the cooling water is changed at the same time. A workshop can ask for between € 100 and € 400 for the replacement. Labor included.

How many thermostats does my car have?

Usually, each car has only one thermostat. Top-of-the-range coupes sometimes have two thermostats, one of which can also be installed in the oil circuit.

Can I continue to drive with a faulty thermostat?

If the thermostat no longer opens, you should only continue driving if there is no other way. You should always keep an eye on the temperature. With an open thermostat, continuing your journey is no problem at all.

How many degrees does a thermostat open?

Thermostats typically open between 80 ° C and 110 ° C, depending on whether the cooling system is unpressurized or pressurized.

Does every car have a thermostat?

Yes, in 1928 the Ford Model A was still delivered without a thermostat. A few years later, however, each car had a thermostat in the cooling circuit.

Which manufacturer should I buy my thermostat from?

Well-known thermostat manufacturers are MAHLE, Hella, WAHLER and Valeo.


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