Furnace Heat and Blower work in On, Not in Auto

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DPhilli23

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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?
 

Fitter30

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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?
Need model# for a wiring diagram. Take a pic of the wiring diagram on inside of blower door. Fan in auto is controlled by a fan control or board in the furnace. When stat is in the on position it runs on high speed not medium which doesn't hurt anything just moves more air. Might have a fan control above burners that looks like this a silver box should have three or four wires going to it. Do you.have a volt meter?
www.supplyhouse.com/White-Rodgers-5D51-35-Fan-And-Limit-Control-Fan-50-To-265-Degree-F-Range-Limit-100-To-300-Degrees-F-Range-5-Element?utm_source=google_ad&utm_medium=Shopping&utm_campaign=Shopping_X_HVAC_X_SSC_ClassA&gad_source=1&gclid=EAIaIQobChMIkMK8--zoiQMVckp_AB02jgNGEAQYAyABEgKN__D_BwE
 
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