Power steering fail followed by engine system failure?

[Fallon1993]

Hey, just after some advice really, I have a focus titanium 1.8 tdci 06 plate about a month ago it completely cut out and wouldn't start and when I tried to start the car the immobiliser just flashed and the display unit had engine system failure on it.... took it to an autoelectrical specialist he said it was the dash cluster so got that send off for repair, thought tht would be it, soon as we got the car back it started fine again drove for a little while then low and behold broke down again this time said power assist failure, turned the car off and now again won't start again, displaying the engine system failure yet again.... took it back to the autoelecrica specialist for him to replace a fuse under the bonnet but he mentioned a strange wire hanging next to the battery so he isolated the wire as he thought that may have been tripping something, now the mystery wire is an official ford wire so almost like someone has had a modification done and then taken off again ?? Now the car ran fine and there were no issues previously before I got an insurance black box fitted? Could that be the issue?? 

At my wits end just want a working car again!! So annoying! 

TIA