The brief
Mr Yong's A3 had a strange DSG fault: a gear wouldn't engage, and after a restart the engagement came back, then it would happen again. He brought it in. That intermittent won't-engage-then-works pattern 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 starts failing the gearbox can refuse to engage a gear, drop into a safe mode, then recover on a restart as the electronics reset, until it doesn't recover at all. A failing mechatronic unit doesn't get better, 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 gears cleanly, which is exactly the won't-engage-then-resets-on-restart behaviour. 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, every gear engaging cleanly, no safe mode, and the fault gone.
The outcome
Every gear engaging cleanly, smooth, crisp DSG shifts, no safe mode, no won't-engage fault, and fresh fluid and a fresh filter. The A3 went home with the gearbox sorted. A failing mechatronic unit only gets worse until a gear won't engage at all, so changing it, coding it in and relearning the adaptations put the gearbox back where it should be.
