What is an AdBlue System?
The blue fluid modern diesels use to clean up their exhaust.
Simplified animation — not to scale.
In plain English
AdBlue (technically Diesel Exhaust Fluid, or DEF) is a clear liquid made of urea and demineralised water that's sprayed into the exhaust of most diesels built since around 2015. It chemically breaks down harmful nitrogen oxides (NOx) into harmless nitrogen and water vapour, helping diesels meet modern emissions rules.
"Like washing-up liquid for your exhaust — a tiny squirt cleans the dirty stuff before it leaves the car."
How it works
A separate tank under the car holds the AdBlue. A small pump and metering valve inject a precise mist into the hot exhaust stream, just before a special "SCR" (Selective Catalytic Reduction) catalyst. There, the urea reacts with the NOx and converts it to safe gases. If the tank runs empty or the system faults, the car will warn you for many miles — and eventually refuse to start until it's topped up or repaired.
Signs of trouble
- ⚠AdBlue warning light or "No restart in X miles" countdown
- ⚠White crystal deposits around the AdBlue filler cap
- ⚠Limp mode or refusal to start once tank is empty
- ⚠AdBlue tank reading empty when you've just filled it
- ⚠Fault codes for NOx sensor or AdBlue injector
Top-up £15–£30; full system repair £300–£1,500
Always get a written quote. Prices vary by car, region, and parts brand.