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
cars 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 £90+VAT to fit the part and £80+VAT for the part! stuff that!