Yes if you have 120v going to the transformer on the black and white you should have 24v on the yellow wires coming from the transformer You can also put the door back on and turn the furnace on. Pull the thermostat from the base plate and measure between the r and g terminal or the r and c terminal if a wire is hooked up to the c terminal and see what you have. I’m waiting for a -40 degree freezer to come down to temp so I’ll be on this for a while to help if need be. And just a good FYI he transformers they sell at Home Depot 90% of the time don’t have enough va to work