The brief
The Polo had gone flat: down on power, hesitating and stumbling when you put your foot down, a rough idle, the odd misfire, and fuel economy slipping. He brought it in. That's the classic worn-spark-plug picture. The plugs are what light the fuel in each cylinder, and they wear, the gap opens up and the electrode rounds off, until the spark gets weak and unreliable. A weak spark means an incomplete burn: misfires, the rough idle, the hesitation on acceleration as the engine tries to deliver power smoothly and can't, and wasted fuel. They're a wear item, so when they're done, they're done, and a fresh set sorts it.
The diagnosis
A check confirmed it, misfire counts logged and the plugs pulled and inspected: worn electrodes, the gaps opened up well past spec, exactly what causes a weak spark. The coil packs and the rest of the ignition side checked out, so it was the plugs. That's a full set, you don't change one plug, you do all of them together so every cylinder is firing the same, and you fit the correct heat range while you're at it.
The work
The old spark plugs were removed and a full set of new genuine VW-spec plugs fitted, gapped to spec, each one torqued correctly so it seals and sheds heat properly. The coil packs were checked and reseated, and the misfire codes cleared. A road test confirmed a steady idle, no misfires, no vibration, and the power back.
The outcome
Smooth idle, clean pull through the rev range, no hesitation, no vibration, and fuel economy back where it should be. The Polo went home running properly again. Worn plugs only get weaker and make the engine work harder for less, so a fresh set put the power and the smoothness back, the cheap maintenance job that pays for itself at the pump.