Bad/Stuck Car thermostat: Symptoms (Explained)

Failing thermostats can disturb the engine cooling system. It can cause overheating or overcooling issues in the engine. Most of the time when the thermostat fails, it is stuck in an open or closed situation.

In the last article, we discussed in great detail how the thermostat actually works. Now in this article, we will discuss the stuck open or stuck closed thermostat along with its symptoms.

Bad or Stuck Car thermostat

Stuck open and Stuck closed thermostat:

A stuck open thermostat indicates that the main/primary valve of the thermostat is stuck in an open situation.

This causes coolant to continuously circulates through the radiator even at a lower temperature.

The below figure indicates the effect on the engine cooling system in a vehicle when a thermostat remains stuck open.

Coolant flow with stuck open thermostat

The thermostat is said to be in a stuck closed condition when the main valve of the thermostat is stuck in a closed situation. Due to this, the coolant is not allowed to circulate through the radiator even above the set temperature.

The below figure shows the coolant flow with a stuck closed thermostat.

Coolant flow with stuck closed thermostat

Symptoms of Stuck open thermostat:

1] ‘Check Engine’ light turns on: When the thermostat remains in a stuck open condition, it continuously circulates the coolant through the radiator. Due to the continuous cooling, it takes much time to attain optimum engine temperature. Therefore the ‘check engine’ light glows with error code P0128.

2] Temperature indicator on the dashboard takes much more time to rise than normal.

3] Lowered gas mileage: Due to the continuous cooling, the engine operates below the optimum temperature. It results in incomplete combustion of fuel, so the engine burns more fuel. Therefore it lowers the fuel economy.

4] Poor interior temperature heater performance: The heat from the engine coolant is utilized to warm the air inside the car. Thus the lowered coolant temperature results in poor heater performance.

5] Coolant under the radiator cap swirls even at lower temperatures: When the engine is in cold condition, open the radiator cap and then start the engine. If the coolant in the radiator is swirling, it indicates the thermostat is stuck in an open condition.

Symptoms of Stuck closed thermostat:

1] Temperature indicator goes up fast: Stuck closed thermostat does not allow coolant to circulate through the radiator. As a result, the coolant never gets cooled. It causes the temperature of the engine to rise faster.

2] Overheating of the engine: Due to the stuck closed thermostat, the engine overheats faster. It may cause ‘Check Engine’ light to become ON with the engine error code P0217.

3] Coolant leaks: Due to the stuck closed thermostat, the hot coolant can leak from the thermostat housing, coolant hose, radiator, etc.

4] Abnormal sound from the engine: As the coolant does not circulate through the radiator, the temperature of the coolant reaches the boiling point. It creates strange noise due to the boiling of the coolant.

5] One hose warm and one hose cold: When the engine becomes hotter, check the inlet and outlet hoses of the radiator. If one of the hoses is much cooler than the other, then it indicates that the thermostat is stuck closed. This method makes it easy to guess a stuck closed thermostat.


  1. How do I know that the Thermostat is failing?

    Some of the symptoms of a failing thermostat are as follows:
    a] Engine overheating or overcooling
    b] Coolant leakage
    c] ‘Check Engine’ light ON
    d] Lowered fuel economy

  2. What are the reasons for the Car thermostat to fail?

    Some reasons behind the failure of the thermostat are as follows:
    Overheating, Rust due to improper cooling fluid, Weakened springs, etc.


Thermostat image by POWERNATION youtube channel

Pratik is a Graduated Mechanical engineer. He enjoys sharing the engineering knowledge learned by him with people.

Leave a Comment