Fan Failure GTN 625

Had the "Cooling Fan Failure" message yesterday shortly after takeoff while planning a day of Vx/Vy data gathering. Reset the breaker (unplug then plug it back technology) with the same message. Left the CB out and returned to the field. After returning I removed the GTN and the fan on the back of the tray and pinned out the 12v + and - for the GTN and Fan as well as the RPM (yellow lead) for the fan. Powered it up on the bench and everything ran fine. The fan ran at an intermediate level ( I can't measure RPM) and no message for 10 minutes. Powered the fan independent of the GTN and it ran full speed. It seems to me the system ran as planned since the GTN was on the bench and not especially hot to require full fan speed. My thought here is it might be the small plastic plug for the fan on the back of the tray but I read the message will come on if the fan doesn't run as fast as the GTN wants. Has anyone had this message? I measured the voltage of the yellow lead on the fan while running at 0.072Vdc. I cannot find a reference for the correct voltage anywhere or if it is even a proper datapoint to measure. Any help would be appreciated.
Thanks,
David

My 650 has had that message

from day 1 (almost 5 yrs)! Apparently the tach sensor (yellow wire) is screwed up --- I have verified, several times, the fan is working fine and never had an issue.

Ron