AdBlue (DEF)

AdBlue is the European trade name for diesel exhaust fluid (DEF): a clear, non-toxic solution of about 32.5% high-purity urea in demineralised water, standardised in ISO 22241. It is stored in its own tank, usually with a blue filler cap next to the diesel filler or in the boot.

The fluid is not a fuel additive and never goes into the diesel tank. A dosing module sprays it into the exhaust ahead of the SCR catalyst, where it releases ammonia that reacts with nitrogen oxides. Consumption depends on engine and driving, typically a few litres per thousand kilometres.

AdBlue is sold in canisters and at many fuel station pumps. Because the urea solution slowly breaks down when it is hot, store it out of direct sunlight and avoid keeping opened containers for very long.

A pump draws fluid from the tank, a heater keeps it liquid in frost (it freezes at about −11 °C), and an injector doses it in proportion to the NOx the engine produces. NOx sensors before and after the catalyst tell the control unit whether the system is working.

When the engine is switched off, many systems pump the fluid back from the lines into the tank so that it cannot freeze in the injector. A short pump noise after shutdown is therefore normal.

When the tank runs low, the car warns you with a countdown. Once it is empty, or the system detects a fault, most cars will not restart until the problem is fixed, because the engine would otherwise exceed its emission limits. Use only fluid that meets ISO 22241 and keep it clean.

Spilled AdBlue leaves white crystals and can corrode metal and paintwork, so wipe spills off with water. Never top up with water or additives: contaminated fluid can damage the pump and the SCR catalyst, which are expensive to replace.