Thanks Flash and all...
The contacts / terminals were corroded. I couldn't get a good volt or ohm reading yesterday when I looked into it. I sand- papered the lugs and put new FM connectors on the wires. It may have been the cause of the fault. The disc may have been stuck - it ohm'd when I tapped it...worked fine when I restarted.
Its a THT 1221 / new THT2436 - I got the part # from the manual. I'm on the the way to Trane to pick one up.
The HX looked good. But I have to get an inspection mirror to get a look at more of it.