AdBlue explained: what it does and what goes wrong

AdBlue is a liquid that many modern diesels use to cut the nitrogen oxides in their exhaust. It sits in its own tank, separate from the fuel, and needs topping up from time to time. When the level gets low the car warns you, and if the tank runs empty the engine may not restart until you refill it. The system can also develop faults that are not cheap to fix. For a used buyer, the questions are simple: does the car use AdBlue, has the system had problems, and is it all still fitted and working.

What AdBlue does

AdBlue is a solution of urea in purified water. It feeds a system called selective catalytic reduction (SCR). A small dose of AdBlue is sprayed into the hot exhaust, where it turns into ammonia. In the SCR catalyst, that ammonia reacts with nitrogen oxides (NOx) and turns them into nitrogen and water.

NOx is the pollutant that matters most for diesel cars. The Euro 6 standard cut the NOx limit for diesel cars from 180 to 80 mg/km compared with Euro 5, and SCR with AdBlue is one of the main ways makers meet it. That is why AdBlue is common on Euro 6 diesels, and why it matters for clean air zones that ask diesels for Euro 6. Our guide to Euro emissions standards explains the limits.

Not every diesel uses AdBlue. Some use other ways of cutting NOx. The handbook says which, and a car with AdBlue has a separate AdBlue filler, often with a blue cap, next to the fuel filler or elsewhere on the car.

Range warnings and the no-restart rule

A car with AdBlue tracks how much is left and warns you as it runs low. Usually you get a message with the distance you can still drive, and the warnings become more insistent as the range falls.

Take those warnings seriously. Cars with AdBlue are built so the driver cannot keep ignoring an empty tank, and on many cars, once it runs out, the engine will not restart after you switch off. The handbook explains what your car does.

AdBlue is sold in bottles and at some fuel pumps. Use the fluid the handbook specifies, and never put it in the fuel tank. If it goes into the fuel tank by mistake, do not start the engine and call for help.

What goes wrong

The AdBlue system has more parts than people expect: a tank with a level sensor, a heater (AdBlue freezes at around minus 11 degrees Celsius), a pump, a dosing injector in the exhaust, NOx sensors and the SCR catalyst. Faults that come up on used diesels include:

• Heater and pump faults. These often show as an AdBlue or emissions warning, sometimes with a countdown to a no-start.
• Crystallisation. AdBlue can dry and leave white crystals that block the injector or lines. White crusting around the filler or under the car can be a sign.
• Sensor faults. A failing NOx sensor can make the system think it is not working.
• Contaminated fluid. Water, diesel or the wrong fluid in the AdBlue tank can damage the system.

We do not give repair prices, because they vary widely with the car and the garage. Some repairs mean replacing the whole tank and heater unit. Get a written quote for the specific car and ask for the invoice for any past work.

Emulators and delete kits are illegal

Some firms sell AdBlue "emulators" or "delete" software that turns off the system and fools the car's computer. Do not buy a car that has one fitted, and do not fit one.

• Using the car breaks the law. Regulation 61A of the Road Vehicles (Construction and Use) Regulations 1986 covers vehicles first used on or after 1 January 2001. It does not allow a vehicle to be used on a road if it fails the emission limits that apply to it because of an alteration to the engine or exhaust, or because emission control equipment is not in good and efficient working order.
• DVSA checks lorries for cheat devices at the roadside. From 1 September 2018 DVSA staff across Great Britain began checking lorries for emissions cheat devices. Drivers caught have 10 days to remove the device and repair the emissions system, or they can be fined £300 and have the vehicle taken off the road. That scheme is aimed at lorries, but it shows how seriously tampering is treated.
• It fails the MOT. The MOT inspection manual makes emission control equipment fitted by the manufacturer that is missing, obviously modified or obviously defective a major defect. An engine malfunction indicator lamp that is lit or does not work is also a major defect.

If a seller says the AdBlue system has been "deleted", "removed" or "emulated", walk away. The same goes for a removed diesel particulate filter, which our DPF guide covers.

AdBlue and the MOT

The MOT does not measure NOx. For a diesel car the emissions test is a smoke test. What the tester also looks for is emission control equipment that is missing, obviously modified or obviously defective, and an engine warning light that is on or does not work. The manual says a leak of the fluid required for selective catalytic reduction is not a reason for failure, but a leak is still worth fixing. Our guide to the MOT emissions test explains the rest of the test.

Checks when you buy a diesel with AdBlue

1. Turn on the ignition and check the warning lights come on and go out, including the engine management and any emissions lights.
2. Look at the dashboard messages for AdBlue or emissions warnings, and for a range countdown.
3. Look under the AdBlue cap and around the filler for white crystals.
4. Ask about repairs. Has the heater, pump, injector, tank or NOx sensor been replaced? Ask for the invoice.
5. Read the MOT history for emissions failures and advisories. Our guide to reading MOT history helps.
6. Ask whether the system has ever been disabled and walk away if the answer is yes or vague.
7. See the car and ask a mechanic to read any stored fault codes if you are unsure.

If you are still deciding between fuels, our guide to whether to buy a used diesel puts AdBlue alongside DPFs, clean air zones and tax.

Frequently asked questions

What happens if AdBlue runs out?

On many cars the engine will not restart once you switch off with an empty AdBlue tank. The car warns you well before it gets to that point, so top up when the warnings start.

How often do you need to top up AdBlue?

It depends on the car and how it is driven. The handbook gives a guide, and the car tells you when the level is low.

Is it illegal to remove AdBlue?

Using a car on the road with its emissions equipment removed or disabled, so it no longer meets the limits that apply to it, breaks regulation 61A of the Construction and Use Regulations. Missing or obviously modified emission control equipment is also a major defect at the MOT.

Can I put AdBlue in myself?

On most cars, yes. Check the handbook, use the fluid it specifies, fill only the AdBlue tank, and never put it in the fuel tank.

What does an AdBlue warning light mean?

It usually means the level is low or the system has a fault. A low level is fixed by topping up. A fault warning, especially with a countdown, needs a garage soon.