Mercedes AdBlue fault repair

Mercedes-Benz diesels badged BlueTEC use selective catalytic reduction to cut nitrogen oxide emissions. A separate tank holds AdBlue, a urea solution, which is pumped into the exhaust and turns harmful gases into nitrogen and water. You will find it on many C-Class, E-Class, GLC, Vito and Sprinter models, especially Euro 6 vehicles from around 2014 onwards.

When the system detects a fault it does not just light a lamp. The dashboard shows a message such as AdBlue: No start in 500 miles, and the countdown keeps going. Once it reaches zero, the engine will not restart after you switch it off. That makes an AdBlue fault urgent even though the car still drives normally.

The common failures are well known. The heater inside the AdBlue tank can fail, so the fluid freezes or cannot be measured properly. The pump and level sensor in the tank can stop working. The dosing injector in the exhaust can crystallise and block. The NOx sensors before and after the catalyst can fail and report that the system is not working. Poor-quality or contaminated AdBlue can cause several of these at once.

Topping up is not always the answer, and replacing parts until the light goes out is expensive. We read the fault codes, check the fluid quality, test the heater, pump and sensors, and look at the dosing injector before recommending a repair. On some models the heater, pump and sensor are only sold as one tank assembly, and we will explain that clearly before any work starts.