AdBlue (DEF in English) is the registered trademark of urea used in the SCR (Selective Catalytic Reduction) systems of modern diesel engines. Composed of 32.5% pure urea and 67.5% demineralised water, it is injected upstream of the SCR catalyst where it reacts with the exhaust gases converting NOx (nitrogen oxides) into nitrogen and water vapour.
Why it exists
The Euro 6 (from 2014) and subsequent regulations drastically limit NOx emissions from diesel engines. Without SCR + AdBlue, modern diesel engines would fail to meet legal limits.
Common problems
- Faulty sensors: the AdBlue quality sensor is notoriously fragile
- Crystallization: AdBlue crystallizes at low temperatures
- Broken SCR pump: repair cost €600-€1500
- Refill counter: after In Italy it is illegal for road use (it violates anti-pollution regulations) - acceptable only for export or track use. Technically the modification disables the monitoring of the level, quality and dosages in the ECU file.
Consumption and autonomy
The average consumption of AdBlue is approximately 1-1.5 liters per 1,000 km, but increases with aggressive driving or severe SCR calibrations (Euro 6d). The tank (10-24 liters depending on the model) generally covers 8,000-15,000 km between top-ups. When the level drops below the minimum threshold, the panel shows a countdown of the remaining starts: once exhausted, the car will not start again until it is topped up.
Diagnosis and maintenance
SCR faults often cause a progressive reduction in power (derate) before blocking. For the diagnosis, the real values of the NOx sensors upstream and downstream of the catalyst and the declared quality of the urea are read: inconsistent values indicate a faulty sensor rather than poor AdBlue. After replacing the SCR pump or injector, an adaptation with a diagnostic tool is almost always necessary.
In practice
For those who manage a workshop, AdBlue should be treated as a consumable subject to expiration: it degrades in the heat and should be stored between -5 and 25 °C. It is best to top up with an ISO 22241 certified product and avoid contaminated funnels, because impurities and incorrect concentrations damage the SCR injector. If the warning light is on, act before the ignition lock is triggered to avoid stopping the vehicle and the subsequent unlocking procedure with diagnostics.