What is the AdBlue (SCR) system?

What AdBlue actually does

AdBlue is a urea solution stored in its own tank with a blue filler cap. The vehicle injects a fine mist of it into the hot exhaust, where it converts harmful nitrogen oxides into harmless nitrogen and water inside the SCR catalyst.

It is what allows a modern diesel to pass Euro 6 emissions limits, which is why the vehicle takes the system failing so seriously.

The parts involved

Any one of these failing can trigger warnings and, eventually, a non-start countdown.

• ✓AdBlue tank with level, quality and temperature sensors
• ✓Tank heater, so the fluid does not freeze in winter
• ✓Metering or dosing pump and pressure lines
• ✓AdBlue injector in the exhaust
• ✓Upstream and downstream NOx sensors
• ✓SCR catalyst itself
• ✓The SCR control module and its wiring

Where it goes wrong

The two most common failures we see in the workshop are NOx sensors and crystallised injectors. Both throw emissions codes that look alarming and both are straightforward once properly diagnosed.

Winter brings tank heater faults, and high-mileage vans often need a pump. Cheap or contaminated fluid causes crystallisation, so it is worth buying sealed AdBlue from a reputable supplier.

Looking after it

Top up before the warning appears rather than after, use sealed fluid, and do not let the tank run dry. If a warning appears within a few hundred miles of a genuine top-up, book a diagnostic instead of adding more fluid.

AdBlue specialists in Salford

We diagnose and repair AdBlue and SCR faults on any make from our Salford workshop at M7 1UL, covering the whole of Greater Manchester. Fixed prices from £180, most vehicles the same day.

Common questions