The brief
The E200's fuel filler flap had stopped behaving: it wasn't locking and unlocking with the central locking, sometimes stuck shut so you couldn't fill up, sometimes not securing at all, with a buzz from the rear quarter when it tried. He brought it in. That's the fuel flap lock actuator. On these the filler flap locks and releases electrically along with the doors, driven by a small actuator behind the rear quarter panel. The little motor and the latch wear, so the flap stops responding reliably, which is the stuck-shut or won't-secure behaviour and the buzzing as the actuator strains. A flap lock that's started failing only gets worse, until it leaves you unable to refuel or with the flap insecure, so it needs replacing.
The diagnosis
A check confirmed it, the fuel flap lock actuator was worn, not driving the latch cleanly, which is exactly the unreliable behaviour and the buzz. The wiring and the central locking checked out, it was the actuator itself. That's a replacement, you don't rebuild the actuator on the car, so the call was a complete fuel flap lock unit, fitted and tested with the central locking.
The work
The rear quarter trim came off to reach the fuel flap lock, the failed actuator removed, and a new genuine Mercedes-spec lock unit fitted, connected up, the latch and linkage adjusted so the flap catches and releases cleanly, and the trim refitted. The flap was cycled with the central locking to confirm it locked and released every time. A check confirmed the fuel flap locking and unlocking reliably with the doors, opening when it should and securing when it should, with no buzz.
The outcome
A fuel flap that locks and releases every time with the central locking, opens when you need to fill up, secures otherwise, and does it quietly. The E200 went home with the flap sorted. A failing flap lock only gets less reliable until it strands you at the pump or leaves the flap insecure, so changing the actuator put it right before it became a problem.