Heating, Air Conditioning, Fridge, HVAC/Gas Furnace Fails to Light


I have a old Heil gas furnace, model NUGE100ag02.  The pilot light started turning on and off, never firing the burners.  Based on my internet research, the most likely problem was the flame rod (thermocouple, flame detector, whatever you'd like to call it).  I replaced it (the pilot, ignitor, and thermocouple are a single unit)
.  Now the pilot stays on, but the burners never light off.  No power is going to the main gas valve.  I checked the flow switch on the exhaust blower and it's fine.  I jumpered the 24v from the output of the flow switch to the lead to the gas valve and it lit off.  To me it sounds like the relay panel is the only thing that could be wrong.  Is there something else I should check?
Thanks for your help.

How old is this unit?  Does it have an electronic board with a flashing light.  The flashing light could be a clue as to the problem.  The problem could very well be with the control board.  I would recheck all of the wiring and use your meter to troubleshoot the problem until you find it.


Heating, Air Conditioning, Fridge, HVAC

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Craig HVAC Expert


I have been in the HVAC field for the past 18 years. I can help with most HVAC questions. I work on commerical buildings for the most part, and have yet to find anything I could not troubleshoot and repair, when repairable. I work on small 1 ton units to a 2500 ton chiller. Troubleshoot air flow, elect, and control problems. I attend regular classes to keep up with the latest and greatest.


I work on commerical sites, hospitals, gov't buildings. I can troubleshoot just about anything in the HVAC business.

5 years trade school, VFD training classes, Liebert factory training, some York and Trane factory training.

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