The brief
Raphael's A4 had started misfiring, the engine rocking and stumbling with the check engine light on. He brought it in. A misfire means a cylinder isn't firing cleanly, and left to run it dumps unburnt fuel into the exhaust, hurts economy, and can cook the catalytic converter, so it needs sorting at the source. The most common cause of a misfire is the ignition system: each cylinder has a coil that builds the spark and a plug that fires it. The coils heat-cycle and wear, and when one starts breaking down the spark goes weak or intermittent, so that cylinder misfires and the engine shakes. The plugs wear too, the gap opening with age, which makes a weak spark worse. When you find a bad coil the smart move is to do all the coils and the plugs together, since they've all done the same miles.
The diagnosis
Diagnostics confirmed the misfire and pointed to the cylinder, and a check found a failed ignition coil, with the rest tired from the same mileage and the spark plugs worn, the gaps opened past spec. The injectors and the rest of the engine checked out, it was the ignition side, the usual culprit. So it was a full ignition coil set and a full spark plug set together, every cylinder getting fresh hardware so they all fire the same.
The work
The old ignition coils and spark plugs came out, and a full set of new genuine Audi-spec coils and a full set of new plugs went in, the plugs gapped and torqued, the coils seated properly. The misfire codes were cleared and the engine's adaptations reset so it relearns on a clean ignition system. A road test confirmed a steady idle, no misfires, the light staying off, and smooth power through the rev range.
The outcome
Smooth idle, clean pull through the revs, no misfires, no warning light, and fuel economy back where it should be. The A4 went home running properly again. A failing coil only takes the others and the plugs down with it the longer it's left, so doing both as full sets put the ignition right in one go.