The brief
The Q3 came in with a misfire on one cylinder, and the symptoms all matched.
The engine was rough at idle, the kind of rough you feel through the steering wheel at a red light. Cold starts were taking longer than usual to settle into a clean idle. And the check engine light was steady on the dash.
All four signs point at ignition trouble: one cylinder not firing properly on every combustion cycle.
Ignition trouble breaks down into two suspects, the spark plug or the ignition coil that fires it. Working out which one is the actual culprit is part of the job, because swapping the wrong part wastes the customer's money and does not fix the car.
The diagnosis
Scan tool first. It localised the misfire to a single cylinder and gave us a misfire counter, so we could see exactly how often it was firing wrong.
The quickest way to confirm whether it is the coil or the plug is a swap test. We moved the suspected coil over to a known-good cylinder and watched the scan tool. The misfire moved with the coil, which meant the coil itself was the fault, not the cylinder it had been on.
That is the cleanest diagnostic outcome you can get. No guesswork, no parts-cannon.
While we had the engine cover off, we pulled the spark plugs across all cylinders for a look. They were past their wear spec, with the electrodes rounded off and the gap opened up a touch. So the right call was to fit a new coil on the affected cylinder, and at the same time do the full plug set, because plugs at this wear stage tend to fail next.
The work
Engine cover off, coil packs exposed. The failed coil came out cleanly, since the diagnostic had told us which one it was.
In went a new Audi-spec coil on the affected cylinder, seated and clipped down.
Then the plug set. Each old plug came out, and a new one went back in, gapped to the factory specification and torqued in carefully. Spark plugs do not like being over-tightened, and they do not like being under-tightened either. Each one gets the manufacturer's torque, no more, no less.
Coils reseated, harness reconnected. We cleared the stored faults on the scan tool, then watched the live data through a few idle cycles to confirm the misfire counter was reading zero across every cylinder.
The outcome
Idle was smooth and steady, with no roughness coming through the steering wheel. Cold start the next morning was crisp and settled into a clean idle within a second or two, the way the car should fire.
No misfires registered on the scan tool over the road test. Check engine light cleared and did not return.
For the owner, the Q3 is back to running cleanly through the rev band. And by doing the full plug set rather than just the one coil, the chance of being back in next month with a different cylinder misfiring is much smaller. That is the cheaper outcome in the long run, even if the parts list is a bit longer today.
