The brief
Cindy's Audi had a passenger door that would open sometimes and not others, the kind of fault that's only a nuisance until it's the day you need that door and it won't budge. She brought it in. The door lock is a little motorised mechanism inside the door that does the latching, the central locking and the release. It has small plastic gears and contacts that work every time the door opens or the car locks, thousands of cycles a year, and over the years the gears wear, the motor gets weak, or the contacts go dirty, so it works intermittently, opens with a second try, or stops responding to the central locking. A worn lock actuator doesn't recover, so it gets replaced as a unit.
The diagnosis
The door card came off for a look inside. The passenger door lock actuator was worn, the mechanism not latching and releasing reliably, which is exactly the open-sometimes behaviour. The handle, the latch cable, the switch and the wiring were fine. That's a door lock replacement, the whole actuator, rather than trying to coax worn plastic gears that will only get worse.
The work
The door was opened up, the old lock actuator disconnected and removed, and a new genuine Audi-spec lock fitted, the rods and cables set so the handle and the lock button work properly. The central locking was tested to confirm it locks and unlocks on this door with the rest of the car, and the door's vapour barrier and card put back properly so there are no rattles or leaks. A check confirmed the door opens first time every time, latches firmly, and locks and unlocks with the central locking.
The outcome
The passenger door opening first time every time, latching firmly, and locking and unlocking with the rest of the car, and the door back together tight. The Audi went home with the lock sorted. A failing lock actuator only gets less reliable, so replacing the whole unit fixed it properly rather than waiting for the day it jams shut.