AdBlue (DEF)

AdBlue is a precisely specified solution of urea in demineralised water, carried in its own tank and metered into the exhaust stream ahead of the Selective Catalytic Reduction catalyst. There it converts nitrogen oxides into harmless nitrogen and water vapour. It is not a fuel additive and never goes into the diesel tank.

Consumption runs at a few percent of diesel consumption, varying with engine load and duty cycle. That makes it a real but secondary line item — significant enough that a fleet should track it, small enough that it is often forgotten until a vehicle runs dry.

Running out has consequences beyond emissions. BS-VI vehicles progressively derate engine power when the AdBlue tank empties, and will eventually restrict the vehicle to a limp mode. Quality matters too: contaminated or out-of-specification fluid can damage the SCR catalyst, which is an expensive component to replace.