Just an update: the drive select system still unreliable. Came back working for a few months but out again. Had it checked and seems to be a module fault rather than anything a software update can rectify.
Forgot to provide an update on this. Got it fixed sometime back. It was a valve or relay sitting in water in the plenum chamber that was shorting the ADS system. I discovered that the AC wasn't working anytime the ADS system went AWOL. Both seem to be connected somehow electronically.