A tip for Sony CDX-A changer.
I found the message was from last year, so don't know if you've fix the problem. If not, maybe here's a tip, as I had been fiddling with the antique Sony A bus stuff for a while.
Generally the A series of changers are great solid and realiable stuff, so the problem might be in the connection. Check the plugs into the modulator, both from the changer and the remote control. Try to unplug it and plug it in and fasten it with with the 2 little hooks in the plug.
Don't know if this makes sense to you.
Gerald