I have gotten a 'undefined fault" error several times in my 600 miles so far on my experia. It always happens at a stop light. No diagnostic code in the menu. Just the red exclamation. I turn it off and back on and all is well. 
I doubt this has anything to do with your issue, but my 2012 Zero S had the same sort of problem (no warning light, though). It was solved by replacing the throttle assembly with an updated part.
