Nest C wire weirdness

[Chuck37]

My nest has been having battery issues recently, so I started looking into the C wire thing.  I have an unconnected blue wire going to the thermostat, and it measures 33V between red and that wire.  It seems a little high, but not crazy, so I hooked it to the C connection.  When I did this, the thermostat recognized the C wire as connected but claimed no voltage was visible at the red anymore.  When I remove the connection from C, it seemed back to normal.  What could this mean?

[Ryan_G]

Hey Chuck37,

Thank you for reaching out and we're sorry for the delay.

Could you take a photo of your wiring so that we can further check on what's happening?

I hope to hear from you soon.

Regards,

Ryan

[Chuck37]

I resolved it.  That blue wire was not common/ground.  I found an unused wire and grounded it at the AC unit.  This seems to be working fine now.

[Ryan_G]

Hey Chuck37,

Thanks for the update and it's great to hear that it is up and running now.

If you have any other questions, don't hesitate to get back to us.

Regards,

Ryan