I found somebody else with a similar problem, they said,
"I have a 2001 mk1 ford focus fixed this repair myself on a very cold saturday morning, here was the problem...
1) when cold/frosty or 1st thing in a morning/late at night, when i went to lock the car using the key fob (the usual one press to lock the doors, 2nd press to deadlock with flashing indicators) it sounded like it was trying to lock but unlocked straight away...
kind of sounded like "clunk...clunk" as if something was catching and preventing locking?
then when i tried to use the key in the drivers door lock, the same thing happened.
2) while in the car sometimes when i went to lock all the doors from the inside drivers door, the same thing "cluck...clunk" etc
3) however all the doors would lock if the passenger door inside lock was used. and could lock all the doors using the key in the passenger door lock.
never really happened during the day or when it was warmer?
so i did a bit of internet reading and was pretty sure something was sticking in the door mechanism, probably the solenoid because i read if it feels resistence it unlocks??? which would explain why the doors would lock from the passenger side.
tried the recommended drowning the lock in WD40 - doesnt work.
a ford garage agreed with me but wanted ÂŁ90+VAT to fit the part and ÂŁ80+VAT for the part! stuff that!
went down to a local scrap yard, removed a good condition mechanism from a similar focus. not an issue to do this at all, as this method uses your orgininal lock barrel and door handle etc so no issues with reprogramming of keys.
the mechanism cost ÂŁ25 and took me like 3 hours to replace. and it all works beautifully!! no changing fuses, batteries or programming keys!! this is the solution to the problem!!
it is quite a complex job and allthough im only 21 with fairly capable mechanic skills i think any competant DIYer could do it?
you ideally need a haynes manual, some TORX screw bits, phillips screw bits and thats about it really and a lot of patience!!!
what i did was to take my haynes manual with me to the scrap yard and practise the whole removal/refitting before i touched my precious focus!!
if attempting this repair i would suggest once the whole mechanism has been removed, spend 15minutes looking at what goes where and ideally take photos/draw pictures as once you start taking things apart its easy to forget where things go? "