Dead P38 4.0se

Hi new to this forum. My P38 has had the door locks replaced twice and have failed again. The becm locked out so I had it rebuilt by a firm that advertises on the net, (twice) I also changed the fuse box as recommended by some sites but now the car is dead again. I have re-built two series 3's from the chassis up and my disco gear and transfer boxes but this has stumped me. Electronics are a dark science to me. Any ideas?
Harleybazza

i would say probably an immobiliser problem with the becm, becm and engine ecu may have to have the security code re-syncronised