The brief
The Touran had crossed the 60,000 km mark, the point the plugs are due for replacement, and the engine had started to let the owner know: a check engine light on the dash and a rougher feel under load. He brought it in, and the timing was right, those plugs were overdue.
The spark plugs light the fuel in each cylinder, and they wear a little every time they fire. Past their interval the gap opens up and the spark gets weak, and a cylinder starts missing, which the engine computer logs as a misfire and flags with the light. A set that's done 60,000 km and started misfiring is a set that needs changing, which is exactly the job that came in.
The diagnosis
The scan came back with misfires logged on a couple of cylinders and the multiple-cylinder code, plus the malfunction lamp on. The plugs came out and went on the bench: the electrodes were worn right where you'd expect at this mileage, the threads rusted, no oil fouling and no other underlying problem, just a tired set past its life.
Reading the plugs that way is useful in itself: the wear was even and clean across all four, which tells you the rest of the engine is healthy. So it was a straightforward plug-only job.
The work
Four new VAG-spec spark plugs went in, gapped as supplied and torqued to the manual's figure so each one seals and conducts heat properly. The coil packs were reseated on top with the harness clips clicked home, and the stored fault codes cleared.
Then the engine was run at idle to confirm it was firing cleanly on all four with no stumble.
A road test confirmed the misfire was gone.
The outcome
Steady idle, no misfires, no fault codes, and the check engine light out.
The Touran went home good for the next interval, with the engine confirmed healthy from the plug-read. Plugs are a wear item, and once they're past their life they drag the whole engine into rough running and worse fuel use, so a fresh set was the cheap fix that sorted all of it.