The brief
The CLA200 was at the recommended interval for an automatic transmission fluid service. The owner had done his reading and knew the trick with ATF: it's far cheaper to change it on schedule than to wait for the gearbox to start shifting badly and then go chasing wear after the fluid has cooked. So he booked it in proactively, with nothing actually wrong.
Automatic transmission fluid does a lot of jobs at once: it's the hydraulic fluid that works the gear changes, the coolant inside the box, and the lubricant for everything spinning in there. It breaks down with heat and miles. Refreshing it, with a clean filter, is one of the best-value things you can do for the most expensive part of the drivetrain.
The diagnosis
The first check is the old fluid itself. Drained out, it had darkened with age the way you'd expect at this mileage, but there was no metal glitter in it and no burnt smell, the two things that would point at real wear inside the box. The magnetic strip on the pan had only light fines on it, which is normal.
So the verdict was a clean one: a healthy gearbox that just needed what it was in for, fresh fluid and a new filter. A standard interval service, not a repair.
The work
The transmission pan came down, the old fluid was drained out completely, and the magnetic strip on the pan was cleaned off. A new Mercedes-spec pan with the filter built into it went on with a fresh seal, and the box was refilled with the correct ATF spec, set to the right level at the warm temperature it's measured at.
Then the gearbox adaptation routine was run on STAR, so the transmission controller wiped its old shift map and re-learned a clean one on the fresh fluid.
A road test confirmed it shifted cleanly through every gear pair.
The outcome
Shifts are smooth across every gear pair on the road test, no flare, no hunting under load.
The CLA200 went home with the gearbox reset for another long service interval and the most expensive part of the drivetrain protected. That's the whole point of doing ATF on schedule: it's a small, planned cost that keeps a very big, unplanned one off the table.