Why does the engine needle fluctuate up and down?

[JoelRose]

The reasons for the engine needle fluctuating up and down: if the water temperature sensor fails or malfunctions, or if the engine cooling system leaks. If the water temperature sensor fails or malfunctions: When the engine ECU detects that the output signal from the water temperature sensor is unreliable, it will intervene and use a substitute value. This is also to protect the engine, resulting in an unstable reading on the car's water temperature gauge. It may also be accompanied by difficulties in cold starting the car and poor acceleration. Engine cooling system leak: Check if there is a leak in the engine cooling system and if the water pump is functioning properly. If it is not working correctly, it can cause the car's coolant temperature to fluctuate between hot and cold.