I have bought a new 'case' for my 2002 206HDi car key. I have swapped the case only (button pads had fallen out), and kept the circuit board & the metal key itself. BUT when I insert the key in the ignition the dash displays "immobiliser fault".
If I swap the parts back into the original casing, then the ignition responds OK. I assume there is some sort of "ID chip" in one half of the casing that is 'read' by the car column ignition area?? How do I correctly repair / replace the key fob please?

Trev, September 2013

Be careful the chip is separate from the other electronics i.e. the battery etc. It is contained in a little rectangular box in the casing remove it carefully and add to the corresponding box on your new key fob. My car started straight away when I did this.

Bill, May 2014

Put the "chip" into the new casing. You've probably left it in the old casing by mistake. It is quite tiny so it is easy to miss.

ChicksFan, September 2013

It will need to go onto a correct computer to code your new key in

cdc, September 2013