I have a Trane TEM6AOC36H31SAA with a XL824 thermostat.

It's not working. The Nexia app says "not connected".

I switched the breaker at the panel and nothing. When I left it off for 5+ mins and switched on, the thermostat fired up for 30 secs then blank again.

I put bleach and water plus another quart of water down the drain line.

There's a red lamp on connected to the drain pan....I don't recall ever seeing it lighted before.

Nexia says it's bad thermostat or service call. That means $200+.

Anything else I can try?

* I don't know why those attachments are so big in size? Sorry

I'll admit to knowing next to nothing about this thermostat, but in general I would pull the thermostat off of its sub-base. The sub-base is screwed to the wall and the part that you see and interact with snaps onto the sub-base. Note that there might be wires going to/from the sub-base, so don't just yank the thing off--be careful.

Once that's done, I'd check for 24 VAC between the R and B terminals, and/or look for bad connections. I'd see what color wires go to these terminals and then go to the furnace and check at that end as well for bad connections. Assuming you do have a good, solid 24 VAC then it might be the thermostat itself. The fact that it comes on briefly and then goes off has me concerned.

Sorry I can't offer any more than this.
WallyH