A replacement remote keyfob from Ford, is quite expensive, so he was keen for me to try and fix it.
To get the key apart, use a small screwdriver inserted into the slot at the back to pry the remote section out of the key yoke. The two halves of the remote section then simply unclip.
Should you decide to order a new remote from Ford, you'll find the part number on the remote section just below the slot. The RFID chip, which deactivates the immobiliser appears to be located in the half containing the battery, so if you don't have the two keys required to reprogram the immobiliser to accept the RFID in a new remote, swapping the battery compartment over would probably work.
By now I'm wearing an anti-static strap, rather than risking zapping the electronics. They only cost about 2 quid from somewhere like dealextreme, but don't buy "wireless" ones, as they are a con, and don't work.
Gently pop the top peg out of the slot in the PCB to release it.
With the PCB removed, you can see the three miniature switches.
Testing my dad's ones with a meter revealed that the lock button had a partial short, enough to slowly drain the battery, but not normally quite enough to trigger the doors to lock
The cause was corrosion inside the switch, the debris from which, was creating the short. It must have got moisture in, although my dad is certain that the key has never got wet. The key for his previous Focus went through the wash cycle on at least one occasion that I know of, but never developed a fault, the seal on this one clearly isn't so good.
The switches come apart quite easily, by using a jeweller's screwdriver to pop off their metal shells. They are a miniature leaf switch, one leaf acts as a contact, the other increases the force required to operate it. The one shown on the left was black, but cleaned up quite well
As a temporary fix, I unclipped the metal shells, scraped off the worst of the corrosion from the contacts and the silver plated leaf on each switch with a jeweller's screwdriver, and cleaned them with isopropyl alcohol. This restored the remote to full working order, if you are lucky that might be all that is needed, but because of the very poor condition of the lock switch, I decided to source some replacements.
The switches are 2 x 6 mm KSR subminiature tactile switches made by C&K. From measuring the pressure required to operate the originals, I reckoned the 4.5N KSR251GLFS is the best match. You might even find you could swap the leaf switch and buttons over, rather than unsoldering the base, providing you can get the contacts in the switch base nice and clean.
I'm afraid I don't have any pictures of soldering the replacement switches in, as I passed the job on to my brother; he's vastly better at soldering fiddly smt components than me. He tells me that he used a craft knife to separate the solder joints while heating it with his iron.
When you come to soldering the new switches on the solder pads on the switch are gull-winded so should draw the solder in, the tricky bit is having a steady hand so as not to move them, or if you are clumsy like me, you might find it easier to solder them if you use a plastic spring clip to hold them in place.
After reassembling and testing the key, you should fix the PCB and its cover firmly in place, as even a small amount of play can cause the battery contacts to bend and eventually lose contact with the pads on the PCB when pressing the buttons.. So I'd suggest gluing it in place, or as I have, to make it easy should I ever need to take it apart again, I stuck a small thin square of foam rubber (I used a strip of stick on rubber feet, but something like draft excluder might be thin enough) on the PCB cover so that it presses on the centre of the battery.
The key is now working fine, and cost less than a tenner to fix, with a few spare switches left over, should it ever fail again.