Fiat Panda 2010 169 4x4 1.3 No 4 Wheel Drive DTC C1900 & C1901

Hi all,
Sorry it's been a while!

I've got a 2010 Panda 4x4 1.3 Multijet diesel that says "4 wheel drive unavailable" and 4x4 warning light on.
No faults in engine ECU. 2 faults stored in ABS, C1900 & C1901. No idea what these mean.
I've got the latest version of Multiecuscan.net and it just says sensor open circuit.
There's a solenoid on the rear diff, I measured 7 ohms across it, so that's not open circuit.
I am assuming the 4x4 system is part of the ABS system.
Anyone seen this before, have any info on fault descriptions and or schematics?

Thanks is advance.
Greg