Part number 4H0 121 671D is not a generic part number, but a specific HVAC heater control solenoid valve used in various Volkswagen Group vehicles (including Audi and Volkswagen brands).

Although the car water pump has a simple structure, it is the power source for the engine cooling system and is crucial to the engine’s health. Understanding its function, working principle, and common faults helps car owners to identify potential problems in a timely manner and perform proper maintenance, thus avoiding major issues and protecting the expensive engine.