Sign of malfunction | Reason/Features |
| A.The temperature, according to the instrument readings, increases slowly, the operating temperature is reached very slowly, the heating is weak. | Thermostat valve plate is stuck in the open position (for example, due to scale); access to the radiator remains open all the time. The engine runs too long in a cold state. Although this may not lead to serious malfunctions in a short time, it is still necessary to replace the thermostat as soon as possible. |
| B.The temperature, according to the instrument readings, is close to boiling or the warning light is on despite the normal coolant level. The radiator and lower radiator hose are cold. | The thermostat valve plate is blocked (for example, due to the leaky walls of the glass of the heat-sensitive filler). Do not continue driving under any circumstances, this may cause serious damage due to engine overheating! Replace the thermostat. |
Note. Recommendation: if the engine overheats on the road due to a faulty thermostat, you can only move on with a tow truck or after replacing the thermostat. If you decide to replace it, wait until the coolant cools down. Find a suitable container to drain the coolant. If you cannot find a container, you can pour clean water into the cooling system for a while. However, do not forget to bring the concentration of the antifreeze additive up to standard as soon as possible!