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Question DetailsAsked on 12/9/2017

My pilot lite is on but burners are not lighting already changed the thermocouple on 30 year old Lennox furnace

Blower kicks on an pilot is lite new thermaocouple an not to tight clean filters but burners not lighting on a 30 year old Lennox gas furnace

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Assuming the thermocouple is properly inserted in the receptable in the gas control valve, and tht it is properly in the pilot flame, most likely cause is a faulty thermocouple or a failed gas control valve. I have heard a LOT of complaints recently (from individuals and HVAC techs) about new thermocouples coming in defective - I only use Honeywell and have not had that problem, so if it comes to calling a pro for $75-150 or trying another new thermocouple, I would start there. If new one works return the other one as defective, otherwise keep the extra one as a spare for next time you need to swap it out.
On a 30 year old furnace a failed sensor is a possibility, but that old not likely to have many (if any) sensors on it. Of course, dead thermostat or dead batteries in it, someone accidentally moved/reset the temperature setting or flipped it to A/C or OFF setting, or broken wires (usually at the ends, not mid-run) could also be the case and easy to check for. And of course check (if you have volt-ohm meter) for power at the furnace - that it did not trip out, have a transformer failure, etc. Oh - blower is working, so not that. Also - double check the cover is on right and that any spring-loaded cover safety switch is in place and that it will "close" when the cover panel is put on - most furnaces have an open-panel safety switch that prevents it firing up if the access panel is removed ornot fully seated. Here are links to a pretty fair number of similar questions with answers on troubleshooting which I hope help - otherwise a Heating and A/C Contractor is the Search the List category for find well-rated and reviewed techs to look into this.

Answered 2 years ago by LCD

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