The brief
The Touran had lost a noticeable chunk of power under acceleration, and the engine shook through the steering when load came on. Eventually the check engine light came on. That combination, weak pull plus a shudder under load plus a warning light, is a misfire, and it needed a proper diagnosis rather than guessing at parts.
A misfire is a cylinder that isn't lighting cleanly. When one or more cylinders skip, you lose the power they'd contribute, the engine runs unevenly so it shakes, and the engine computer notices and flags it. The usual culprits are the spark plugs, which wear out, and the ignition coils that sit on top of them, which fail with age. On a high-mileage engine showing misfires, both are suspect.
The diagnosis
The scan came back with misfires logged across all four cylinders, the random and multiple-cylinder code as well, plus a stored throttle and pedal position code. The misfires were the live fault and the cause of the symptoms.
The plugs came out. Mileage put them well past 60,000 km and the electrodes were worn past spec, so they were due. With a misfire spread across every cylinder on a high-mileage engine, and the coils being the same age and the same wear item, the sensible call was to refresh the whole ignition side, a fresh set of plugs and a fresh set of coils, rather than chase one cylinder at a time.
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 power was back, the vibration under load was gone, and no misfires returned.
The outcome
Power back, no vibration under load, no misfires on the road, and the check engine light out after a drive cycle.
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, reliable ignition across all four cylinders rather than another misfire a few thousand kilometres down the road.