Following a P1125-5 error (Critical Malfunction System 1) and some reading up, it looks like the crankshaft sensor will be the first port of call.
However, before I got on that, was looking a lot like boost pressure sensor and EGR valve/sensor checks would be on the cards also.
One concern I have is that I've already removed the ECU, but just read that it will need resetting. Has anyone done this before? Would a Tech2 or Op-com system be able to do it...?