Our A4 started to run on 3 cylinders, I removed all spark plugs and noticed #2 was a darker colour than the rest. What I have done so far is switch coils which made no difference, switched spark plugs with the same result and performed a compression test on the dead cylinder that showed me that is also not the problem. I then ran the engine with the coil from #2 cylinder attached to a spark tester and discovered there is no spark. My question is how do I check for the trigger signal at the coil ?

 

Turned out to be a defective ignition control module, replaced it with a new one and now it's running on all four cylinders.