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Trying to add common wire to furnace for smart thermostat. Looking at wiring diagram, it looks like common wire off transformer is blue, switches wire color to white (via wire cap), then back to blue (via connector block), and connects to terminal TH on the ignition control module (SP-735A). Is this understanding correct? Can I just add a wire off the TH terminal (or off combustion blower relay) to get a common?

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I am not so sure that the Common of the transformer connects to terminal TH of the ignition control module. It looks more like the TR terminal and it looks like another terminal is labeled TH.

I would encourage you to locate the wire join point depicted below and add your C wire into the existing 3 wire join to make it a 4 wire connection. If you are lucky this may just be a simple wire nut that you can remove. If it is a crimped wire termination then you have a little more work to do cutting it off and then re-stripping the wires to allow use of a proper sized wire nut.

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