The brief
The Golf's DSG had gone awry: jerky shifts, hesitation between gears, a gearbox warning on the dash, and dropping into a limited-power safe mode. He brought it in. That points at the mechatronic unit. The DSG is a dual-clutch automatic, and the mechatronic unit is its brain and hydraulics in one: the control module, the valve body and the solenoids that engage the clutches and select the gears. It's a known weak point, the electronics and the solenoids wear, and when it fails the gearbox loses its smooth, precise shifting, jerks, and drops into a safe mode, with the warning up. A failed mechatronic unit doesn't recover, so it needs replacing, coded to the car, the adaptations relearned, and the DSG fluid and filter done with it.
The diagnosis
A diagnostic scan pulled the fault to the mechatronic unit, the solenoids and the control side not engaging the clutches and gears cleanly, which is exactly the jerky shifting and the warning. The clutches and the gearbox internals checked out as far as could be assessed, it was the mechatronic at fault. That's a mechatronic replacement, you don't rebuild it on the car, so the call was a complete unit, fitted, coded to the car, the adaptations reset and relearned, and the DSG fluid and filter renewed.
The work
The old mechatronic unit was removed, the DSG fluid drained, and a new genuine VW-spec mechatronic unit fitted with the DSG filter renewed, then refilled with the exact DSG-spec fluid to the proper level procedure. The new unit was coded to the car, the clutch and gear adaptations reset and relearned through the proper procedure, and the stored faults cleared. A road test confirmed smooth, crisp DSG shifts through the range, no jerk, no hesitation, no limited mode, and the warning light off.
The outcome
Smooth, crisp DSG shifts, no jerkiness, no hesitation between gears, no limited-power mode, and no gearbox warning, with fresh fluid and a fresh filter. The Golf went home with the gearbox sorted. A failed mechatronic unit only gets worse and takes the DSG's shifting with it, so changing it, coding it in and relearning the adaptations put the gearbox back where it should be.
