I’m trying to install a smart thermostat and my furnace does not have a C terminal. The thermostat has an unused blue wire, and I connected it to what I thought was a common, but the unit is not powering on. Any help would be appreciated.
The wire with the "wago" connector seems to be the common. check it with a multimeter between it and the red one coming from the transformer (it should read 24V).
My idea is that t-stat wire is too thin to make good contact in wago connectors. Try a standard nut (or fit the blue in the same hole as a side of the thicker wire you've already tapped into).
Also check continuity of the blue wire in the wall, it may be broken, or have incomplete splices inside the wall.