Did you find what your blinking light codes indicate ?

Thanks! I’ll check the drain lines. I assume you mean the two that empty into the floor drain?

As to the blink light codes, I’m seeing 21 green blinks to one red blink

I don’t see this sequence mentioned in the manual or furnace stickers (maybe it’s an “ all OK” code).

I do find things like this online:

https://www.justanswer.com/hvac/679ui-trane-xv95-green-light-blinks-21-times-red.html

A good suggestion. I've had water issues with my furnace in the past.....caused very erratic burner function. I GENTLY blew out the drain lines as well as the water separator and a couple sensors located inside that had hoses connected to them. I did that based on directions from my buddy who owns an HVAC company and installed my unit. This year I had him come and do a tune-up on my now 10 yr old unit....he tore most of the internals out....cleaned everything,blower,burners,water separator,hoses/lines,etc....surprising how much water and dirt can accumulate. We also installed a fancy wifi/Alexa T-stat...So far it's working perfect and the house is very comfortable. Your mileage may vary.

Yes, there are actually two separate ones.

One is for the air conditioner condensate, the other is for the furnace itself. That's the one I was referring to. You need to completely disassemble the drain lines and make sure all the fittings and passageways are clear and unobstructed. IIRC, the forced draft has to be able to blow/draw through those drain lines freely or the draft sensor will shut the machine down.

And then again, your furnace problem could be something entirely unrelated to this?

Thanks so much everyone!

Problem finally solved!

Located the flame sensor. Didn’t realize I had to unbolt the burner box to get to it.

Replaced with new sensor, but did not resolve problem.

Ended up pulling the thermal switch, and bridged the two leads with alligator clips.

Immediately fixed the problem (burners not igniting).

Coincidentally. The new thermal switch to is supposed to arrive later today.