My 2003 Mk1 For Focus, which has a new battery, starter motor and solenoid, has intermittent starting problems. The flashing immobiliser, red light, on the dashboard, indicates that the immobiliser intermittently fails to recognise both of the ignition keys. Although this happens some five out of twenty starts, the twenty second wait for the immobiliser to deactivate can be embarrassing.

I propose to replace the immobiliser antenna and attached control box. My question is, if I buy a replacement unit from a scrapyard, should I expect any specific difficulties, such as both my keys being no longer recognised at all. Any advice regarding replacing an immobiliser would be greatly appreciated. It would also be convenient to use the immobiliser antenna from a 1.8 diesel on my 1.6 petrol car.