The brief
The Golf had crossed 120,000 km, which on this engine is the spark plug interval. The owner brought it in proactively for the change rather than waiting for a misfire or rough idle to show up. The right call, plugs are a wear item and once they're past their life they drag the whole engine into rough running and worse fuel use. 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, until a cylinder starts missing under load. That's the power loss, the hesitation and the harder starting that a tired set eventually causes. Done on schedule, a fresh set keeps the combustion clean and the engine smooth, and heads off the misfires before they start.
The diagnosis
The four plugs came out and went on the bench. The electrode wear was right where you'd expect at this mileage, no fouling, no oil contamination, no signs of any underlying problem, just a set that had done its time. A diagnostic scan confirmed no fault codes stored. Confirming clean, even wear like that is useful in itself: it tells you the rest of the engine is healthy and the change is being done at the right time, before any misfires set in. So it was a straightforward plug-only job.
The work
Four new genuine VW-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 engine run at idle to confirm it was firing cleanly on all four with no stumble. A road test followed to confirm it idled steady and pulled cleanly through the rev range.
The outcome
Steady idle, smooth pull through the rev range, no misfires, and no fault codes. The Golf went home good for the next plug interval, with the engine confirmed healthy from the plug-read. Plugs are cheap and the job is quick, so doing it on schedule is the easy way to keep the engine running its best and avoid the misfires that come from leaving them too long.