At what point is it acceptable to refill the DEF tank?

Refilling the Diesel Exhaust Fluid (DEF) tank at the ¼ tank level is the most advantageous choice for several reasons. Maintaining the DEF tank at this level helps ensure that the vehicle's emissions control system functions optimally, as DEF is critical for reducing nitrogen oxides in diesel exhaust.

When the DEF tank is allowed to become too low, it can lead to performance issues and potentially trigger warning lights or reduced engine power if the system runs out completely. Refilling at the ¼ tank ensures there is enough fluid available to prevent running out unexpectedly, providing operational continuity while eliminating the rush or inconvenience of filling it at lower levels.

In addition, frequent checks and top-offs at this point can aid in preventing issues related to the crystallization of DEF, which can occur when it sits in the tank for an extended period, especially if the tank is allowed to run very low or empty. By adopting the practice of refilling at the ¼ tank level, it enhances operational efficiency and vehicle reliability.