SCR and AdBlue Basics
How selective catalytic reduction cuts NOx, and the faults that follow bad urea or a failed dosing system.
Loading…
What you will learn
- Explain the SCR principle
- Describe urea dosing and quality
- Recognise common SCR faults
Advertisement
Selective catalytic reduction injects a urea solution (AdBlue) into the exhaust, where it reacts in the SCR catalyst to convert nitrogen oxides into harmless nitrogen and water. The dosing is controlled by the ECU using NOx and temperature sensors.
The system is sensitive to urea quality and to its own sensors and heater. Contaminated or diluted urea, a failed dosing pump/injector or an incorrect NOx reading can trigger warnings, power limitation and even a countdown before the vehicle refuses to restart.
| Component | Job | Common fault |
|---|---|---|
| Urea tank & pump | Stores and doses AdBlue | Crystallisation, blocked filter |
| Dosing injector | Sprays urea into exhaust | Clogging, no spray |
| NOx sensors | Measure NOx before/after | Ageing, contamination |
| SCR catalyst | Converts NOx | Contamination, overheating |
AdBlue is not a fuel or a cleaner — never add anything else. Handle urea per the manufacturer instructions and do not disable the system; it is an emissions control device.
Key points
- SCR converts NOx using dosed urea.
- Quality and quantity of urea matter more than many expect.
- Read NOx and temperature data before condemning parts.
Loading…
Key points
- SCR injects urea to convert NOx
- Urea quality and level matter
- Faults often limit power or count down a range
All values shown are training examples. Real thresholds depend on the manufacturer, engine, temperature and system — always confirm with the vehicle service documentation.
Advertisement