I've had the same problem with my doblo since I bought it a few months ago. So, feeling adventurous this afternoon, I thought I'd have a go at tackling this problem myself.
Just finished,and everything's working again now.. Only took me about an hour, (and I'm normally mechanically inept, so if I can fix this, anyone can!).
First had to remove the inner panel from the boot door. 10 screws.
With the panel off, and the boot open, I pressed the key to see how the locking mechanism works. There's a little white solenoid which pushes and pulls to lock/unlock. In my case, the little white push-rod was still connected to the lock, it just appeared that when the solenoid "pushed" to close the lock, it didn't "push" all the way. If I gave it a little help (by pushing on the push rod on the lock myself, in time with the solenoid), then the central locking would lock.
So..
I unhooked the solenoid, and removed it from the door (1 screw).
I then removed the entire boot door-handle mechanism (3 allen screws, and un-hook another push rod to boor door catch mechanism).
With the entire boot door handle mechanism removed, I tried to work out what was making the mechanism so stiff that the solenoid couldn't lock it.
Basically there was just a load of greasy gunk in the locking mechanism.
I sprayed WD-40 liberally all over the lock mechanism, and wiped away as much of the greasy gunk as I could. I then worked in a lot of 3-in-1 oil.. opening and closing the lock with the key, and applying more oil, until the mechanism became light and smoother.
Then started re-assembling. Fastened door-handle mechanism back on (3 allen screws). re-attached push-rod to lock catch at the bottom of the door. Screwed solenoid back on, and re-attached push-rod from solenoid to lock.
Voila, the solenoid was now able to push and pull correctly to lock and unlock the boot with central locking.
re-attached the boot door inner panel, and done !
I like that the doblo's mechanically simple enough to work on like this... With all of my previous cars, I'd have just taken it to the dealer and paid £75 per hour labour for a job like this.
