How AdBlue Works
What the AdBlue system does.
AdBlue is diesel exhaust fluid used by the SCR system to reduce NOx emissions on modern diesel cars, vans, trucks and plant.
AdBlue is not a fuel additive
AdBlue is stored in its own tank and is injected into the exhaust system, not the diesel tank. It is a mixture of urea and deionised water, often called DEF or diesel exhaust fluid.
The system is fitted to many Euro 6 diesel vehicles to reduce nitrogen oxide emissions. The vehicle monitors AdBlue level, quality, dosing pressure, temperature and NOx sensor readings.
Reading the AdBlue diagram
The engine produces exhaust gases that contain NOx. Those gases move through the exhaust system toward the SCR catalyst.
The AdBlue tank stores diesel exhaust fluid. The pump sends it through the dosing line to the injector, where it is sprayed into the hot exhaust stream.
Inside the SCR catalyst, the AdBlue reaction helps convert NOx into nitrogen and water vapour. NOx sensors before and after the catalyst report back to the ECU so it can check whether the system is working.
1. Diesel exhaust leaves the engine
After combustion, exhaust gases contain NOx. The SCR system treats those gases before they leave the tailpipe.
2. AdBlue is dosed into the exhaust
The ECU controls an AdBlue pump and injector. It sprays a measured amount of fluid into the exhaust stream.
3. The SCR catalyst does the work
Inside the SCR catalyst, the treated exhaust reacts and helps convert NOx into nitrogen and water vapour.
4. Sensors check the result
NOx sensors, pressure sensors, level sensors and temperature sensors report back to the ECU. If readings are wrong, warning lights and fault codes appear.
Why AdBlue faults stop vehicles
When the vehicle thinks the AdBlue system cannot reduce emissions properly, it may limit performance, start a countdown or prevent restart. Common causes include low AdBlue level, poor fluid quality, pump pressure faults, tank heater faults, crystallised AdBlue, blocked injectors and failed NOx sensors.
Plant, trucks and vans can be hit especially hard because downtime costs money. Good diagnostics matter before parts are replaced.
See common P-codesCars, trucks and plant
The core SCR process is similar, but the hardware layout changes by vehicle. Cars and vans often use compact tank modules, while trucks and plant may have larger tanks, longer dosing lines and harsher working conditions.
Adblue Wales can help with repairs, fault finding and eligible non-road delete advice.
