What Is AdBlue? What Drivers Need to Know

Every modern diesel truck and most diesel vans have a second filler cap, and what goes in it confuses more people than any other fluid on the vehicle. So, what is AdBlue, what happens when it runs out, and why does putting it in the wrong tank rank among the most expensive mistakes in motoring? Here is the plain-English version.

What AdBlue actually is

AdBlue is a precisely mixed solution of 32.5 per cent high-purity urea in de-ionised water, made to the ISO 22241 standard. It is not a fuel and not an additive: it is injected into the exhaust, where it reacts inside the SCR catalyst, selective catalytic reduction, converting harmful nitrogen oxides into harmless nitrogen and water vapour. SCR and AdBlue are the main reason modern diesels meet the NOx limits of Euro 6.

How much you use

Consumption is a small percentage of diesel use, typically around 4 to 8 per cent for trucks working hard, less for vans. In practice a truck fills AdBlue every few diesel fills, and a van might go several thousand miles per top-up. The dashboard warns well in advance with a countdown of range remaining, and truck stops, forecourts and bunker sites sell it at the pump as well as in containers, something worth checking when comparing fuel card networks.

What happens if you run out

The system does not politely ignore an empty tank, because the law does not let it. First comes a hard warning, then engine power is reduced, and once the vehicle is switched off with the tank empty, it will refuse to restart until AdBlue is added. For an operator that means a vehicle stood at the roadside or a bay for the sake of a ten-pound fluid, so AdBlue level belongs on the daily walkaround check next to fuel and screenwash.

The mistakes that cost real money

• AdBlue in the diesel tank. Do not start the engine. AdBlue corrodes fuel system components, and running it through injectors turns a drain-and-flush into a rebuild.
• Diesel in the AdBlue tank. Equally serious in the other direction: even small contamination can poison the SCR system, and catalyst replacement is painfully expensive.
• Cheap unbranded fluid. Anything not certified to ISO 22241 risks the injector and catalyst. The saving per litre is pennies; the repair is thousands.
• Dirty funnels and containers. AdBlue is fussy about purity, so keep it sealed, out of long sunlight, and handled with clean kit. It also freezes around -11 degrees, which the vehicle’s heated system handles on its own.
• Emulator devices. So-called AdBlue delete kits are illegal on the road, show up at inspections, and put the operator’s licence and repute on the line. DVSA treats emissions cheat devices as a serious offence.

Bringing it together

AdBlue is a simple chemical doing an important job: it keeps diesel NOx legal. Keep the tank topped up, buy certified fluid, never confuse the fillers, and it will never cost you a thought, let alone a breakdown. For more practical guides to running trucks and vans well, browse Insights from Life Internet Services, the group behind TrucksLife and TrailersLife.

Frequently asked questions

What happens if I run out of AdBlue while driving?

You get escalating warnings, then reduced engine power, and once the engine is switched off it will not restart until the tank is refilled. The vehicle will not be damaged, but it will be stranded, so treat the countdown warnings seriously.

Can I use water instead of AdBlue?

No. The SCR system needs the precise 32.5 per cent urea solution made to ISO 22241, and sensors detect incorrect fluid. Water or off-spec mixtures trigger faults at best and damage the catalyst at worst, so only certified AdBlue should ever go in the tank.

Is AdBlue harmful or dangerous?

It is non-toxic and non-flammable, though it irritates skin and eyes, corrodes some metals and kills paintwork finish if left, so rinse spills with water promptly. Store it sealed in a cool place and it keeps for around a year or more.