Thank you Susie and Martin for your help. I've talked to my Thermo tech support and he thinks it's the board.

On Tue, 28 Feb 2017 at 17:05 Susan Carter <[log in to unmask]> wrote:
Indeed, that is correct: it was the XY motor driver board.  After some trial and error, and much remote help from Thermo engineers and Isogeochem colleagues, I was able to diagnose and replace it.  The individual boards are relatively economical.
Many thanks to all who helped with this!