Audi AdBlue and SCR Repair in Cardiff

Two NOx sensors, and knowing which one is lying

These systems use a NOx sensor before the catalyst and another after it. The car compares the two readings to decide whether the AdBlue dosing is actually working. That comparison is the heart of the whole system, and it means a perfectly healthy dosing system will be reported as failing if the sensor watching it has drifted or died.

The downstream sensor is the one that fails most often, and it is the one most frequently misdiagnosed as an AdBlue quality or dosing problem. We look at live data from both sensors and compare them against what the exhaust is actually doing, rather than reading a code and ordering a part. Getting that right is the difference between a sensor and a dosing unit, and your bill reflects which one it turns out to be.

The tank heater, and why faults appear in winter

AdBlue freezes at around minus eleven degrees, so the tank has a heater to thaw it. When that heater or its sensor fails, the fault often shows up only in cold weather and then seems to fix itself when it warms up. If your Audi throws an AdBlue warning every winter and behaves the rest of the year, that pattern is worth telling us about, because it points somewhere specific.