My furnace is a gmp075-3

There is an existing unused Blue wire coming from the thermostat to the furnace area but not attached to anything. (upper blue wire in 2nd image)

Can I just connect the upper blue wire on the outside of the furnace to the lower blue wire going to the screw on the top left of the furnace panel?

The transformer is grounded to the screw on the right of the transformer, do I need to ground my C wire from the Thermostat to the same spot as the transformer or can I just connect the existing blue/c wires as is? (ie grounding the thermostat C wire by just pigtail the two blue wires outside furnace)

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  • Can you post a photo of your furnace's wiring diagram please? Dec 22, 2020 at 3:08

1 Answer 1


Yes that blue wire sticking out of the cabinet looks like it's intended to be used that way. To verify use a volt meter to measure AC volts between the blue and the red wires. It should be somewhere in the range 24-30 volts.

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