The SCR system running on AdBlue is the most common reason for a workshop visit on this engine. The fault can start in several places. The NOx sensor works with a heated measuring cell and ages, and the sensor after the catalytic converter is the most affected, because it sees the most moisture and temperature change. The injector of the dosing module is clogged by crystallised urea, and because of that the dosed amount deviates from what is needed. Crystals are left in the lines as well: on short trips and during long idling the water evaporates out of the solution, and the crystals left behind obstruct the dosing. The electronics of the pump inside the tank can corrode from contact with the urea. The symptoms are the warning light, a jump in AdBlue consumption, a wrong reading from the level sensor and finally limp mode. If the SCR control unit locks up, the car will not even start until it is repaired. The repair is between 400 and 1,200 euro, and it is made more expensive by the pump sitting inside the tank. A component cannot be deduced from a warning message: the diagnosis needs several values looked at together. The fault can start from 60,000 km.