The brief
This A3 had crossed 120,000 km, and a whole cluster of symptoms had turned up at once.
Misfires, rough idle, hard cold starts, sluggish acceleration, and a check engine light. All of them appearing together over a short stretch of time.
When symptoms cluster like that, it usually points at one root cause rather than five separate problems. In this case, the mileage was the clue. Ignition coils have a service life, and around 120,000 km is a common point for the first set to start dropping off.
A coil that is on its way out fires a weaker spark. One weak coil gives you a misfire on that cylinder. Several weak coils give you exactly the cluster of symptoms this car had.
The diagnosis
Diagnostic scan first. It came back with misfire codes across multiple cylinders, not just one, which already pointed away from a single bad coil and toward a set that had aged out together.
We bench-tested each coil individually rather than guess. Three of the four measured high secondary resistance, which is the electrical signature of a coil that can no longer build a full-strength spark.
That made the decision straightforward. Three coils were already failing, and the fourth was the same age and the same mileage. Replacing just the three would have meant the customer was back within a few months for the last one, paying another labour charge.
Full-set replacement was actually the cheaper path here, not the more expensive one.
The work
All four ignition coils came out, replaced with new VAG-spec units.
We reseated the harness clips properly on each one. A coil connector that is not fully clicked home can mimic a failing coil, so it is worth getting that right.
Then we cleared the stored fault codes on the scan tool and took the car out for a road test through a full drive cycle, watching the live misfire counters.
The counters stayed at zero across every cylinder, which is the confirmation that the fix held.
The outcome
Idle smoothed out straight away. Cranking back to a normal half-second from cold. Throttle response sharp again, with no sluggishness off the line.
No fault codes after a full drive cycle, and the check engine light cleared and stayed off.
For the owner, the A3 is back to running cleanly through the rev band. And with a fresh full set of coils, the ignition system is good for another long stretch, rather than picking off cylinders one at a time over the next year.
