What does it mean when the DEF light is on in a truck?

1 Answers
TylerMarie
07/30/25 5:50am
Truck DEF light is the indicator light for the auxiliary water tank. When the light is on, it indicates that the auxiliary water tank is low on water. Below are relevant details about the auxiliary water tank in vehicles: 1. Main function: When the engine temperature is high, the liquid expands, and the antifreeze flows into the auxiliary water tank through the radiator cap. When the temperature drops and the cooling system pressure becomes negative, the system draws the antifreeze from the auxiliary water tank back into the cooling system via the radiator cap; 2. Inspection method: The auxiliary water tank has corresponding markings indicating the liquid level, which can visually guide you on the current water level and whether it is sufficient. It is necessary to regularly check the liquid level of the auxiliary water tank to determine if the tank is low on water; 3. Importance: The auxiliary water tank acts as a supplement to the main radiator and is an important component of the cooling system. The main radiator generates certain pressure under varying temperatures, and the auxiliary water tank helps regulate this pressure.
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