The brief
Louis's A5 was rocking with a misfire and the check engine light on. On a high-mileage engine that combination is the classic sign of ignition coils on their way out, so that's where the diagnosis started. Each cylinder has a coil that builds the high-voltage spark and a plug that fires it. The coils sit in the engine bay through years of heat cycles, and the plastic sockets on them get brittle and crack with that prolonged heat, so the spark goes weak or intermittent and the cylinder misfires. The plugs wear too, the gap opening with age. When you find one tired coil at high mileage, the smart move is to do all the coils, and the plugs with them, since they've all done the same miles.
The diagnosis
Before starting the job the car was lifted for an inspection to be sure there was nothing else behind it, there wasn't, and the check confirmed the ignition coils were failing, showing fatigue with brittle sockets, which is the misfire, the rocking and the light. The plugs were worn alongside them. That's a full ignition coil set and a full spark plug set, every cylinder getting fresh hardware, rather than swapping one coil and waiting for the next.
The work
The old ignition coils and spark plugs were removed, and a full set of new genuine Audi-spec coils and a full set of new plugs fitted, the plugs gapped and torqued, the coils seated properly. The misfire codes were cleared and the adaptations reset so the engine relearns on a clean ignition system. A road test confirmed a steady idle, no misfire, the light staying off, and smooth power through the rev range.
The outcome
A steady idle, clean pull through the revs, no misfire, no warning light, and fuel economy back where it should be. Louis got the A5 back running properly again. The coils all wear at the same rate, so doing the full set with the plugs put the ignition right in one go, no coming back for the next one.