What is the DEF warning light?

[SanBrooklyn]

DEF is the auxiliary water tank warning light. When this light is on, it indicates that the water in the auxiliary tank is nearly depleted, serving as a reminder for the owner to add water promptly. Below is relevant information about the auxiliary water tank: 1. Function of the auxiliary water tank: When the engine temperature is high, the liquid expands, and some coolant flows into the auxiliary tank through the radiator cap. When the temperature drops and the cooling system pressure becomes negative, the system draws the coolant from the auxiliary tank back into the cooling system through the radiator cap. Additionally, the auxiliary tank has corresponding markings indicating the liquid level, providing a clear guide on whether the current water level is sufficient. 2. Working principle of the auxiliary water tank: Thermal expansion and contraction. When the car is running, the rising water temperature increases the pressure inside the radiator, causing some water to flow out through the diversion pipe connected to the upper water chamber of the radiator. When the vehicle is turned off, the temperature of the radiator decreases, reducing the internal pressure, which allows the water in the auxiliary tank to flow back into the radiator tank.