This often cures non starting and unexpected cutting out of the engine!
This is refering to the secondary immobiliser on the car. You will have this if you have a remote key fob, and a small LED in front of the instrument panel. After it has cut out, when you go to restart it, the LED will blink out a fault code. This means your immobiliser is knackered!
Lately, this immobiliser has caused quite a few people problems (me included). If your engine suddenly starts cutting out unexpectedly or not starting, this is often the culprit. It is cause by the solder joints in the immobiliser drying out and not providing a good electrical connection.
To replace the box it will cost you about £160 + VAT plus about £200 labour charge as Ford insist on taking the dash out to do it, which really isn't necessary.
To locate it, remove both stereo units (or one unit and the plastic compartment on some models), and look through the holes to the left using a torch. Mounted vertically and bolted to the plastic panel you will see a flat black box about 6 inches square. It is held in place by 3 tork bolts and they are very difficult to get to with the dashboard in. But it is possible to do it. Look for a bunch of brown wires to locate the box.
Or another way of doing it.... (Credit Peter Devine).....
"When you see the immobiliser. Look carefully at the bracket holding it in place. It is mounted on the tunnel with three rivets. To the left and above the rivets is a vertical section which then reverts to horizontal before reaching the box holder itself. Cut through that vertical section (don't cut any wires) using a pair of bolt cutters or healthy metal shears. The right saw might do the job too. When you have done that it is relatively easy to move the immobiliser and its bracket (free but still attached) to gain access to the torxs and they can be removed. The bracket and immobiliser when separated can be withdrawn through the radio socket. Throw the bracket away. Wrap the immobiliser in foam or rag to cushion it when you reinsert it after bridging the right connections. Refit the radio etc and the job is done. Remarkably easy once you figure out how. But isn't everything. It took a lot of fiddling and head scratching (not to mention cursing, scratched hands etc, etc) to work it out. "
Details of how to bypass the immobiliser