Hannes I’m not familiar with Sigma but believe it might use the Ford embedded PATS 2 immobiliser just like the Duratec. If so I would suggest cleaning fuse number 7 (of 8).
On the Duratec fuse 7 is a heavy 20amp fuse that keeps the ecu memoray alive and the immobilser awake when the ignition is off. The current draw is minute at sub 10mA so the big fuse might just be in need of a scrub up. Worth a shot maybe…. Good luck anyway.
