AdBlue Explained: What It Is, How Much You Use and the Warning Lights

What AdBlue is and why diesels need it

Diesel engines burn with excess air and produce nitrogen oxides, which the strict Euro 6 limits do not allow in large amounts. Most modern diesels therefore use selective catalytic reduction (SCR). The car sprays a measured amount of AdBlue into the exhaust ahead of a special catalyst.

In the hot exhaust, the urea breaks down into ammonia and carbon dioxide. In the catalyst, the ammonia reacts with nitrogen oxides and turns them into nitrogen and water. The reaction works best at about 200 to 350 °C, which is one reason SCR systems control dosing carefully.

AdBlue is not a fuel additive. It never mixes with the diesel and it does not affect power. If it runs out, the car simply refuses to restart; Volkswagen describes this as a required feature of the system, not a fault.

How much AdBlue does a car use?

Wikipedia puts dosing at typically 2 to 6 % of diesel consumption, depending on the system; the rental company Enterprise quotes 3 to 5 % for its fleet. Your own figure depends on the system and on how much diesel the car burns.

Here is what that means per 1,000 km, worked out from the 2 to 6 % range:

Tank sizes vary a lot. Enterprise lists 8 to 33 litres across its rental cars, which is why some cars need AdBlue every 5,000 km and others go 15,000 km or more. Your handbook gives the tank size; the trip computer usually shows the remaining AdBlue range.

To see how fuel itself compares on cost, use the fuel cost calculator with your real consumption figure.

AdBlue warning lights: what each stage means

Every brand uses its own symbols and thresholds, so check the handbook. Volkswagen's UK owner pages give a clear example of how the stages work:

Volkswagen stresses that the red stage "is not a fault, but a required feature of the system". It also says the system then needs a full top-up before the car will restart, so carry enough to fill the tank, not just a few litres.

A different message, such as "AdBlue system fault" or "check emissions system" with a full tank, points to a technical problem rather than an empty tank. Have the fault codes read by a garage before the countdown to the no-start stage runs out.

How to refill AdBlue

1. Find the right filler. It is usually a blue cap next to the diesel filler or under the boot floor. The handbook shows the location.
2. Use fluid to ISO 22241. Containers and pumps carry the AdBlue name or the AUS 32 marking with the standard number.
3. Fill up properly. Volkswagen says a full top-up is needed after the red warning, and the handbook gives the tank size.
4. Wipe up spills. Urea corrodes aluminium, so do not leave it on metal parts.
5. Check the display. If the warning does not clear after a full top-up, ask a garage to read the fault memory.

AdBlue is sold at fuel stations and in canisters at DIY stores, car accessory shops and garages, and sometimes at discount stores, according to Enterprise.

Storage, freezing and contamination

• Freezing: AdBlue freezes at −11 °C. Cars thaw it with coolant or electric heaters, and Euro 6 rules under Regulation (EC) No 692/2008 require the fluid to be available within 20 minutes when starting at −15 °C with a frozen tank.
• Heat: it starts to decompose at around 60 °C. Store canisters in a cool, dry, ventilated place out of direct sunlight, not in a hot boot all summer.
• Containers: because urea corrodes aluminium, keep AdBlue in its original plastic or stainless container.
• Cleanliness: as a precaution, use the canister's own spout rather than a funnel or can used for fuel or oil.

If you accidentally pour AdBlue into the diesel tank, or diesel into the AdBlue tank, do not start the engine. Call your breakdown service or dealer and have the tank drained.

Which cars use AdBlue?

Not every diesel uses SCR, so check the handbook. A second filler cap marked AdBlue, or an AdBlue range in the trip computer, tells you the car has it. Petrol cars, hybrids with petrol engines and electric cars do not use it.

If you are choosing between fuel types, AdBlue is a small extra running cost on top of diesel, servicing and the particulate filter that every modern diesel also has. Our petrol vs diesel fuel cost comparison shows when the lower fuel consumption still pays off, and the diesel car pages list the models in the catalogue.