The brief
The Touran was rough at idle, hesitating off the line, and the owner had noticed fuel economy slipping on the routes he drives every day. Eventually a check engine light came on. That spread of symptoms, the rough idle, the hesitation, the thirst, the light, all points at the ignition system.
A misfire is a cylinder that doesn't light cleanly. When that happens you lose the power that cylinder should give, the engine runs uneven so it shakes at idle and stumbles when you ask for throttle, it burns more fuel because of the unburnt charge, and the engine computer flags it. The parts behind it are the spark plugs, which wear out, and the ignition coils above them, which fail with age. On a high-mileage engine, both are due a look.
The diagnosis
The plugs came out first. Mileage put them well past 60,000 km from the last set, so they were due regardless of the codes, and they were fouled. With the plugs that old and a misfire on a high-mileage engine, the call was to refresh the whole ignition side rather than chase one cylinder, so a fresh set of plugs and a fresh set of coils.
The work
Four new VAG-spec spark plugs went in, gapped to the manual's figure, and four new VAG-spec ignition coils went on top of them. The harness clips were reseated, and the stored fault codes cleared.
Then the engine was run to confirm it was firing cleanly on all four with no shudder.
A road test confirmed the idle was steady, the throttle response was back, and no misfires returned.
The outcome
Steady idle, throttle response back, fuel economy back to normal across the next tank, and the check engine light off and staying off.
The Touran went home running cleanly. Plugs and coils are wear items that age together, so doing the full set in one go means even ignition across all four cylinders rather than another misfire a few thousand kilometres later.