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Hello, I have a gas furnace that is shutting off shortly after it starts when the thermostat is connected. If I bypass the thermostat (connecting red and white wires) the furnace will run perfectly fine. Here is the odd part, after I flip the breaker, and reconnect the thermostat the furnace will work properly with the thermostat for 1-3 days. Then it will shut down again.



The furnace seems to go through the proper startup. However, at the initial steps before I bypass the thermostat, the furnace will start properly and run for 2-3 min, there is a click (or two) and then the furnace shuts off. It will attempt to restart again and run for 2-3 min and then shuts off completely. :vs_OMG:





I have taken the following steps: new thermostat, new filter, opened all the vents, cleaned the blower, checked the flame sensor.
 

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What model is this furnace?
Does it have a diagnostic light?
Before replacing any parts I’d strongly advise to diagnose the unit first. The thermostat could have been checked before replacing it.
 

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TIP: I use a piece of a brown paper bag for cleaning flame sensors. It's a neat material for other delicate jobs. I first started using it years ago on the advice of a Potter Brumfield tip for cleaning/burnishing relay contacts.
 

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Are you using a power stealing thermostat, battery operated, or one that uses the common wire of the furnace.
 

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Okay, and when you bypassed it, did you do that at the furnace, or at the wires where the thermostat is.
 

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Do you have an unusually long thermostat wire?
Have you checked the voltage at the wires with the thermostat disconnected?
Have you checked the voltage at the furnace where the wire connect when the thermostat is off? What type of thermostat did you have before? Was it a simple off/on or was it programmable as well?
Nothing wrong with all of the other things you did as far as maintenance but if you did just one thing at a time it would have helped troubleshooting.
I suspect this is an older furnace. It's "possible" that your transformer is going bad and not allowing enough voltage to pull in the contractor / relays to keep it running.
Get a cheap voltmeter if you don't have one and do some checking.
 
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