Hi, I have an Audi A6 3.0 BiTDI from 2013. Two months ago, my first injector failed. We replaced it, and when I test drove the car, another injector failed. We replaced that one as well, along with all the copper washers, O-rings, and bolts on all injectors. However, when we were about to test drive the car again, yet another injector failed. What could be causing this issue? Or do all the injectors need to be replaced?