One aspect of a vehicle with SCR system is the need for replenishing Diesel Exhaust Fluid (DEF) on a periodic basis. DEF is carried in an onboard tank which must be periodically replenished by the operator based on vehicle operation. For light-duty vehicles, DEF refill intervals typically occur around the time of a recommended oil change, while DEF replenishment for heavy-duty vehicles and off-road machines will vary depending on the operating conditions, hours used, miles traveled, load factors and other factors.

DEF is an integral part of the emissions control system and it must be present in the tank all times to assure continued operation of the vehicle. Low DEF supply triggers a series of escalating visual and audible indicators to the driver or operator. Once the tank reaches a certain level near empty, the starting system may be locked out the next timen when the vehicle is used, preventing the vehicle from being started without adequate DEF.

On-board tanks to store DEF are typically located in the spare tire area of passenger vehicles, while tractor trailers typically have a DEF tank alongside the diesel fuel saddle tank. Proper storage of DEF is required to prevent the liquid from freezing at temperatures below 12 degrees Fahrenheit, and most vehicle DEF dispensing systems have warming devices.