comes out of park and steering unlocks but won't start.turns over like usually.

 

Unable to help. Try posting in the X27 XJS forum, much more traffic.

 

Year of the car your working on ?

Inline 6 or V12 ?

Fully charged battery of 12.75 volts ?

The ignition switch and immobilizer circuit is the same as the inline 6 cylinder X300

Your location in the US should not have a chip in the key or a transponder ring on the barrel

By rotating the key the separately attached rotory switch on the barrel provides grounds that command to close the king relays inside the fuse boxes

Pull the relays out one at a time with the key in the run position and feel for them to click confirming they are getting the correct closing command

The exception is the left engine fuse box where the relay in reality is the horn relay , this relay is a swapping option

My bet is the right engine fuse box as this is the first power to the ECU

Editing