Audi AdBlue Repair and SCR Diagnostics 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.

P20E8 and the codes around it

P20E8 refers to reductant pressure being too low, which means the system could not build the pressure it needs to dose properly. That can be a failing pump, a blocked or crystallised injector, a line problem, or a level and quality issue. The code narrows it down to the delivery side, which is genuinely useful, but it does not name the part on its own. Alongside the generic codes, Audi stores its own manufacturer specific faults with far more detail, and reading those is part of how we get to the answer rather than working through parts one at a time.

Refilling will not clear a stored fault

If the system has logged a fault and started a countdown, topping the tank up does not undo it. The fault has to be diagnosed and cleared properly, and if the underlying cause is still there it will simply come back. If your Audi is counting down to a no restart, do not leave it until the last few miles, because at zero it will not start and it becomes a recovery rather than a drive in.

We do not delete or emulate AdBlue systems. It is an MOT and emissions matter and we will not do it. What we will do is find the actual fault and repair it. See also our AdBlue repair page for all makes and Skoda AdBlue diagnostics.