AdBlue (SCR) explained

Many modern diesels clean nitrogen oxides (NOx) from the exhaust with AdBlue and an SCR catalyst. If the system can’t do its job, the car warns you, and in the end limits the speed or won’t restart.

Updated 16 October 2026

What it does

AdBlue is a solution of 32.5 % urea in purified water, made to the ISO 22241 standard. It has its own tank, often with a blue cap. The car sprays it into the exhaust in front of the SCR (selective catalytic reduction) catalyst. There it turns into ammonia, and the catalyst uses that to turn NOx into nitrogen and water.

NOx sensors in front of and behind the catalyst tell the engine computer how well it works.

AdBlue quality

• Fill only AdBlue, from a sealed container or a pump. Nothing else goes in that tank, not even water.
• Never put AdBlue in the fuel tank: that can seriously damage the engine and the injection system. If it happens, don’t start the engine, or switch it off as soon as it’s safe, and have the car taken to a workshop to drain the tank.
• AdBlue breaks down with heat. Store it cool and out of the sun.
• The car checks the quality. Wrong or diluted AdBlue sets off a warning, and if it isn’t put right soon, the countdown below.

Freezing and the heater

AdBlue freezes at about −11 °C. The system has a heater, and EU rules require the AdBlue to be liquid and usable within 20 minutes of starting at −15 °C. A warning in hard frost that doesn’t go away once the car has run for a while can point to a fault in the heater or the dosing.

The warnings and the countdown

Under EU rules, a newer diesel that uses AdBlue must warn the driver when the level gets low, when dosing is interrupted, when the AdBlue isn’t of the right quality, or when NOx emissions are too high. If nothing is done, a so-called inducement system takes over: the car must not be able to run with an empty tank.

Low level

The first warning comes while there’s still enough AdBlue for at least 2,400 km. On many cars more warnings follow, with the remaining distance.

At the end of the countdown

Depending on the car, the engine won’t restart once you switch it off, or the speed is limited. Where the speed is limited, it’s 50 km/h at most shortly before restarts are blocked.

Faults

A dosing fault, wrong AdBlue or too much NOx can start the same countdown, even with a full tank.

If it only ran empty, the car isn’t broken: it starts again once AdBlue is topped up, though some cars need a reset first. Don’t wait for the end of the countdown.

Fault codes and NOx sensors

P20EE means the SCR catalyst isn’t removing enough NOx, judged by the NOx sensors. Often the cause is the AdBlue itself, a NOx sensor that misreads, or a clogged AdBlue injector, rather than the catalyst.

Read My Car reads the codes and shows the NOx / SCR self-test under emissions readiness, on cars that report it. It can’t reset the countdown or test the dosing: that needs a tool for your make.

• P20EE: SCR NOx catalyst efficiency (AdBlue)
• Diesel particulate filter (DPF) explained

Emulators and switching it off

AdBlue emulators make the engine computer believe the system works while no AdBlue is dosed, and some tuners offer to switch the SCR system off in the software or remove it. All of these are tampering with the emission controls. EU law prohibits defeat devices that reduce the effectiveness of emission control systems (Regulation (EC) No 715/2007, Article 5(2)), and under Directive 2014/45/EU, roadworthiness tests across the EU count emission control equipment fitted by the manufacturer that is missing or obviously defective as a major defect. Have the fault repaired instead.

Questions