Thanks for those suggestions.
I did check the manual in detail and have even reset the unit to factory defaults, but no go.
I am not near a dealer, and I’m moving around so the repair option (if they are interested in doing that) will have to wait. I’ll try the UK as I might be able to organise that in the coming months.
I found those second hand units on eBay earlier today, but they don’t ship to anywhere near me.
I might reconfigure my system tomorrow to use the MFD as the source. This may also solve a problem I posted about previously where there is no heading data at the Hydra display.