Thermostat wiring help - clicking/screen won't turn on

I’m having issues installing my new wyze thermostat and am hoping someone here will be able to help me out. Then I connect the thermostat the screen does not turn on and it repeatedly clicks. Here is what I’ve done:

Old thermostat wiring:

My old thermostat did not have a c wire so I had to use the c wire adapter. My furnace is about 20 years old and the wiring at the control board was a mess. I was able to locate the w, g, and r wires terminals on the control board.

… But there was no c terminal and my thermostat wires were connected via wire nuts outside on the furnace.

I was able to trace the wires from the board to the wire nuts and connect the w, g, r wires to the adapter but I wasn’t sure what to do with the c wire. I connected it to the 24ac terminal on the board (seen on 2nd picture I posted).

I checked the voltage with a multi meter at the new thermostat and was able to measure 27 volts from the red to white, red to yellow, and red to green (labeled c).

Can someone please help point me in the right direction? Thank you!

Since I’m a new user it won’t let me post multiple images on a single post so I’ll add them as replies below.

Control board:

Pre-existing wiring outside furnace:

I disconnected the tape and wire nuts here and connected the c wire adapter to the yellow, red, green (on the right hard to see in the Pic), and white wires going to thermostat. Then connected the c wire on the adapter directly to the 24v terminal on the board.

New thermostat wiring:

can u post a pic of the wired adapter?

also, does the blue wire in this pic go into a the same cable of the yellow (the smaller gauge blue)? I’m thinking the yellow & blue are going to the outside AC condensor. If so, trace the thicker blue wire back to the control board to see if it’s going to the transformer. I bet it’s the C wire. If it is, you would want to connect the C wire of the adapter in that blue wire nut (add it).

Screenshot 2021-04-16 9.15.32 AM


You were right, the blue wire was common. Everything seems to be working now. Thank you!