Hi Jack,
I believe the CGEM mount (like the CPC alt-az) is essentially ‘dumb’ insofar as the computational heavy lifting is all done by the hand set. The mount ‘only’ contains motors and a few circuit boards for controlling them. The error messages you have refer to the failure of the hand set to communicate with the mount.
I had a similar problem with my CPC1100 some years ago:
‘….On power up and after the ‘verifying packages’ message I get a ‘no response 17 invalid model’ message. Pressing ‘back’ produces a ‘no response 16 invalid model’ message. Following advice from Celestron’s website, I’ve tried cycling the power, repeatedly pressing ‘back’, and trying the hand controller in the the two AUX sockets. None of these things have worked. Various people on the web suggest other fixes but these are generally out of my comfort zone (dismantling the ‘scope etc.)….’
I was eventually given the advice (from someone at David Hinds, just before the company’s demise) to leave it for a while then try again. It worked!!!!! The problems occurred during the lock down (5 years ago-ish) and have not returned. One thing I didn’t (couldn’t) try was trying a different handset to see if that worked.
With David Hinds now gone I’m not sure who handles Celestron servicing now. Tring Astro?
Good luck anyway.
Best Wishes,
Trevor