Have you tried adding your own earth to the ecu casing as this is a common problem with them.

Can you talk to the engine ecu if you disconnect the immobiliser ecu.

Can't remember which wires you bridge out at the immobilise ecu so it allows comms with engine ecu as i don't have access to a wiring diagram.

Has it been jump started the wrong way round and spiked the immobiliser ecu as i've had 2 in where this has happened in the last 3 months.

1 needed another immobiliser ecu & key, the other needed an engine ecu.

thanks
lee