A3T not firing on cylinder 4....Help
 

Hi, the car idle like a diesel tractor, I unplugged the coil one at a time while the car was idling and found that there is no change when I unplugged coil nr 3, I replaced the spark plug, no change, swopped coils 2 and 3 around but the fault didn't move to cylinder 2, so the coil seems to be fine, anyone have advise on the next step?

Sorry, it is misfiring on cylinder 3 and not 4 as per the heading

Sorted out, it was the power output stage module! New issue: I removed the cat, saw that you should modify the post cat O2 sensor with non-foulers, the problem is that my car only have one O2 sensor before the cat, what now?