My furnace will only run when the thermostat is set to 'ON' while in the auto setting the burners do not ignite and the blower will not run, but as soon as I change that setting everything works. Any thoughts on where I should pinpoint this down to?

Answer

It sounds perhaps like the low voltage wiring isn't correct. Not something that would just start out of nowhere though unless there is a short in the circuit. This is my guess from afar.

Updated on January 8, 2018

Original Article:

Troubleshooting Common Furnace Problems With an HVAC Expert
By Dan Robbins

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