Gas furnace kept locking out. Different thermostat fixed it.
Posting in case anyone else runs into something similar.
I have a 24 year old contractor grade gas furnace controlled by an Ecobee3 Lite for the last 6 years. A few months ago it started locking out for up to 12 hours at a time without heat. It was short cycling like a flame sensor was going out or something was blocked. Had a tech come out and despite its age they couldn’t find any issues with the furnace. Talked to Ecobee support and they were certain it was the furnace. Eventually they did have me switch some wiring (use W2 instead of W1) and that seemed to help for a few months but then it started again
I decided to try replacing the thermostat with a Honeywell T6 Pro and the furnace has not locked out once in the last month so far. I still hear short cycles I think, but the house has remained heated like nothing is wrong. I’m guessing something really is going out in the furnace and the Ecobee handles it differently, but in case it’s all the Ecobee’s fault I wanted to share my experience.