The brief
A BMW came in at a higher mileage for transmission care, and replacing the automatic transmission oil cooler at this point is good maintenance practice rather than waiting for trouble. Prevention is cheaper than cure, and on the gearbox that's especially true. The transmission oil cooler keeps the gearbox fluid at the right temperature, and on these it runs the fluid past coolant to do it. If that cooler fails internally at high mileage, coolant gets into the transmission chamber, and that's a disaster, the clutch plates corrode quickly, the box fails, and you can end up needing an overhaul. So renewing the cooler before it can fail, along with fresh fluid and a new filter, is the sensible preventive move.
The diagnosis
At the mileage the transmission oil cooler was due renewal as a preventive measure, before it could fail and contaminate the gearbox, and the transmission fluid and filter were due alongside. Nothing was failing yet, this was looking after the box before anything went wrong. So it was a transmission oil cooler replacement plus fresh fluid and a new filter, preventive maintenance done at the right time.
The work
The transmission pan came off, the old fluid drained, the transmission filter renewed, and the transmission oil cooler replaced with a genuine BMW-spec unit. The box was refilled with the correct BMW fluid to the right level at the right temperature, the way it wants it, and the adaptations relearned. The cooling system was checked and topped while it was apart. A road test confirmed crisp shifts, the gearbox temperature steady, and no warning lights.
The outcome
Fresh transmission fluid, a clean filter, a new oil cooler, the gearbox running cool and shifting crisp, and no warning lights. The car went home with the gearbox protected for the long run. Renewing the cooler before it can fail is far cheaper than a contaminated box and a possible overhaul, which is exactly why preventive maintenance is worth doing rather than waiting for the cure.