2000 Ford Focus erratic central locking behaviour.

Easy way to work out which one it is (as I found), open each of the doors and there will be the wiring loom from the door into the vehicle.

On the rubber cover thing at the end it attached to the car body there will be a few little bits that say "push" (not that easy, get close to the rubber and have a look) as if your holding a cd from the edge and squeezing it towards the dentre, this should pop the rubber bit off. Unplug the connectors that are revealed for all the doors.

This will disable all the power locks etc. Plug them in one at a time, test the function (ie use the fob or stick the key in the door, (i started with just drivers door so I could use my key in that one), if it works as it should move on leave it plugged in and and plug the next one in as well, if it works fine carry on untill you hit the one that when you plug it in, the locking starts to malfunction.

You can replace the power lock mechanism, but my trick (as mine was a back passenger door i never use was to leave it uncliped, lock it from the inside - it still locks and unlocks manually and opens fine, just doesnt kill the electrics!), and leave it till I either have the need, or can be bothered to replace the lock mechanism. All my other doors work fine and the cl works all good for the doors connected.

(PS if you have lecky windows, I think unplugging that cable will disable that, but if nothing else its a good way of fault finding the dodgy door(s) and saving yourself a dealer diagnostic fee.)

Hope it helps.