AdBlue (the SCR system) is the exhaust after-treatment on modern diesels that reduces nitrogen oxides. If the system detects a fault or the tank runs empty, the car starts warning you and eventually won't allow the engine to restart. This is a legal requirement, not a breakdown in itself.
The warnings
- "Top up AdBlue" / range X km: the tank is simply running low. Add fluid.
- "AdBlue system fault" or "No start in X km" even with a full tank: there is a fault in the system.
- Fault codes such as P20EE (SCR efficiency), P20E8 (low pressure), P2BAD (NOx too high).
Filling the tank
- Use only AdBlue that meets the ISO 22241 standard.
- Add enough at once – many cars only recognise a top-up after 4–5 litres.
- Never pour it into the fuel tank! If that happens, don't start the engine and call for help.
- After filling, switch the ignition on for a few seconds before starting; sometimes a short drive is needed before the warning clears.
Common faults
- Crystallised AdBlue: white crystals on the injector or in the lines.
- Tank heater or pump: often in winter, when the fluid has frozen (it freezes at about −11 °C).
- NOx sensor: an expensive sensor that can give false readings.
- Poor quality or old AdBlue – it degrades with heat and time.
What not to do
Disabling the AdBlue system (emulators, software) is illegal in Estonia and across the EU, and the vehicle will not pass inspection.
At the workshop
We check dosing quantity, pump pressure, the heater and the NOx sensor readings. After a repair an adaptation or dosing test with a diagnostic tool is often required.
Tips for winter
Keep the tank reasonably full, buy fresh fluid from a busy outlet, and don't store canisters in a hot car boot over summer. If the warning appears on a very cold morning, a longer drive often lets the heater thaw the system.
Enter your fault code into Diagno's AI diagnosis. A trusted mechanic can check dosing, pump pressure and NOx sensors and run the required adaptations.