warwick:

The start up rpm limits are adjustable on the FADEC through the hidden menu, parameters 18 and 19, their values should be;

18 --- 4 (4000 rpms) speed at which starter is shut off on the starting phase
19 --- 2 (2000 rpms) speed at which starter is reenergized on the starting phase

Your values seem to be OK based on your comments

Yet believe your false readings are because of worn brushes of the starter motor, I've had the same happen to me, even if you manage to go through ignition, the false readings will come back during the ramp start, rpms will go past the idle limit and trick the ecu into thinking turbine reached idle, then when the rpms come back down to true reading the ECU will abort the starting phase because after reaching the false "running" condition rpms will go below the minimum limit, then you'll get a "speedlow" message as the reason for the shutdown, all this happens within one second.

I do not think your rpm sensor is malfunctioning, like I said I had the same problem, replaced the starter motor and it's working fine.
I use Great Planes T-370 motor as a replacement, remove the bendix clutch from the bad motor with a GWS gear puller and place on the new motor.

To remove the bendix clutch you first need to remove the pin holding the moving part of the cluth.

This is the replacement motor I use, only $8 bucks...
http://www2.towerhobbies.com/cgi-bin...&I=LXGLK3&P=ML