Diesels with SCR need AdBlue. What the warning means, how to top up and why the car may refuse to start if it runs out.
Looking for more detail? Our full guide covers this topic in depth, with step-by-step advice and sources.
AdBlue (diesel exhaust fluid) is running low or the system has a fault.
What it looks like
A fluid can or the letters AdBlue or DEF. Colour: Amber, turning red.
Common causes
- Low AdBlue level
- Crystallised AdBlue or a faulty pump or sensor
What to do
- Top up with AdBlue to ISO 22241 standard; it is sold at most fuel stations.
- Never put AdBlue in the fuel tank.
- Most cars give a countdown of miles; once it reaches zero, the car may not restart until topped up.
Can I keep driving?
Yes until the countdown ends.
MOT
Not directly, but emissions faults can cause a fail.
Symbols vary slightly between makes. Always check your owner's handbook. General guidance only; if in doubt, call your breakdown provider.
Sources and further reading
Facts checked on 20 October 2026. Spotted an error? Tell us and we will correct it.