start with the front:
take off the wheel and brake disk, clean around sensor area (with a toothbrush or something) before removing with an allen key, be very carefull not to drop any crud inside the hole in the hub when you remove the sensor (including the little O ring) as this may properly screw the ABS/TC up! take out the sensor and give it a wipe as it maybe splattered in grease and put it back.
Trace the wire back up into the inner wing about 18" and you will find an electrical connection, unplug it, pack it with vaseline or silicone grease and reconnect it.
Repeat for all corners...
It's a long shot, but a free one..!
If this dont sort it then get the fault read on the computer...