S4 Jerking & Rough Idle Points to Failing Ignition System

I had similar issues with my 2017 Audi S5, and as someone with moderate experience working on cars, I can share what worked for me. The symptoms you describe sound exactly like what I dealt with, rough idle, jerking during acceleration, and uneven engine performance. After several attempts at diagnosis, it turned out to be a leak in the exhaust system, which was affecting engine performance and triggering the check engine light. The workshop found a crack in the exhaust manifold that was causing these issues. The repair involved replacing the damaged section and checking all connecting parts. Total cost was 265 Euro, including labor and parts. The fix completely resolved the jerking and rough idle issues. Before jumping to conclusions though, could you share: What error codes are you getting? Is there any unusual noise from the exhaust? Have you noticed any decrease in fuel efficiency? When was your last spark plug replacement? These problems can sometimes be related to ignition coils or engine misfire issues, so knowing the specific error codes would help narrow down the cause. A proper diagnosis might also require checking the crankshaft sensor and air filter, as these can cause similar symptoms.

Thanks for the detailed response. The error codes I got were P0302 and P0300, pointing to cylinder misfires. I had my last service at 190396 KM where they changed the oil and filters, but the spark plugs werent replaced then. The timing appears correct and I havent noticed any unusual exhaust sounds. My fuel consumption has increased by about 15% in the last month. After reading your post, I tested all ignition coils and found that cylinder 2 seems weaker than the others. The oxygen sensor readings also show inconsistent values when I connect my OBD reader. The rough idle is especially noticeable after cold starts in the morning. Im leaning towards replacing the ignition coil in cylinder 2 first before looking at more expensive repairs.

Based on those error codes and symptoms, your diagnosis about the ignition coil seems spot on. When I dealt with my S5s issues, I initially thought it was the exhaust, but after getting similar codes (P0300 and P0302), I ended up replacing both the ignition coil and spark plugs for cylinder 2. The oxygen sensor readings you mentioned are likely a secondary effect of the misfire, rather than the root cause. Once I replaced the faulty ignition coil and spark plug, my oxygen sensor readings normalized. Since your spark plugs havent been changed in a while, Id strongly recommend replacing all of them along with the ignition coil. When I did this on my S5, it completely resolved the cold start issues and engine misfire problems. The repair cost me about 180 Euro for parts (4 spark plugs and one ignition coil) and I did the work myself. The 15% increase in fuel consumption matches what I experienced, this improved immediately after fixing the misfire issue. The uneven running during cold starts is a classic symptom of a failing ignition coil, exactly like what I experienced before the repair. Just make sure to use OEM or equivalent quality parts. Cheap ignition coils often fail prematurely and can cause the same issues to return within months.

Thanks for the great advice. I went ahead and ordered a new OEM ignition coil and a full set of spark plugs, figured might as well replace them all since theyre getting up there in mileage. The parts came to 220 Euro total. I installed everything yesterday and the difference is incredible. The engine misfire is completely gone, cold starts are smooth now, and the check engine light turned off after clearing the codes. No more jerking during acceleration either. The fuel consumption seems to be improving already, though Ill need a few more days to confirm. I also noticed the oxygen sensor readings are now stable when checking with the OBD reader. Seems like you were right about that being a secondary effect of the misfire. Really glad I checked here before taking it to a workshop, saved a bunch on labor costs and learned something in the process. Definitely keeping an eye on the ignition coils during future maintenance intervals.