Nest Thermostat No Power M20

[Canuck]

My brand new Nest thermostat worked for a month, and then out of the blue it won't turn the heater (gas furnace) on - I can hear the heater firing up for 1 second and then it shuts down, over and over. Worth noting - it's not the "normal" quiet sound of the heater turning on, but a loud buzz that can be heard throughout the house.

• It's not a heater issue - I reconnected my old thermostat and it works fine.
• All wires are detected (Y-W-G-R)
• Power tests are good  (Vin-35.459, lin: 385, PS: Y-385, Yp-2)
• Battery voltage 3.08 ("good")

No error codes show until it turns the heater on, then it posts the M20 error. As soon as the furnace shuts off, the error clears.

Any idea? Is it just defective, or what?

[Canuck]

Update - seems to have been caused by a short in the W-Y wire. I added a C-Wire and that fixed the power issue, but the AC and Heater then started turning on at the same time. I disconnected the wires from the furnace control board and thermostat and checked for continuity to diagnose the issue. The noise I was hearing was the AC + furnace starting up at the same time.

[Guarcax]

Hi @Canuck!

Thanks for posting in the community.

I'm glad to hear that you were able to solve the trouble. I can see that you were able to find that there was a short between the W and Y wires and this was causing the issue. 

If you need further assistance, you can keep posting.

Regards,

Daniel.