I am trying to troubleshoot a forced air, natural gas furnace. Initially, the pilot was out, so I got it lit, but the furnace would not come in when called by the thermostat. Replaced the thermocouple, same result. Out of curiosity, I kicked the thermostat to ON instead of auto, and the furnace fired up and once heated, the blower kicked on. I figured the Tstat must be bad and replaced, but I am having the same result...nothing in Auto, but fires up and blows when ON. This is an older Armstrong furnace with #101A-G5-220. Simple Honeywell Tstat, nonprogrammable. Where to look next?