Simon,

Engine ECU does ground the control line for the actuation of the starter.

First of all is the START ERROR issue - if the initial stage of Drive Authorisation is not passed then this is displayed on the cluster.

Key, Transponder Coil (clipped on ignition switch), Immobiliser module (located behind cluster) or Engine ECU. Very common failure is the ignition switch on Sprinters so this may not be providing circuit 15 to the Fuse Box??

Obvious first check has to be the spare key (only got one I guess!!!)

Get to the Immobiliser module and check supply voltage and ground to this. If this has lost power all of your complaints are shown!!

Wiring Diagram Essential!!!

Check connections at the Engine ECU - have seen corrosion / loose connections.

Last one I fixed the voltage to the Immobiliser was lost, could not locate the fuse control so bypassed a live and hey presto !!! No Start Error and running.

Can supply diagram but will require vehicle full Chassis number

Good Luck

Darren