Wiring issue

[vanvreede]

• My old thermostat has a R wire (jumped to RC), Blue, White, Yellow and Green Wire. I tried installing the Nest using the Blue wire as suggested for C (Common). I get a e79 error code indicating no power to the common. A volt meter between B and G shows 27 volts. Thoughts?

[sporfrog]

While the color of the wire often indicates the letter, not always. How were things connected before?  What letters?  For example is/was Blue connected to B, or C, or something else?  A photo of the old thermostat connected may help.

[johnCNA]

You should be measuring between R-red and C-blue for your 24v, not B&G.   If all else fails, go look at the furnace connection to be sure that Red=R, White=W, Yellow=Y, Green=G and Blue=C.  You might have to run the setup again on the Nest just to be sure it remembers the correct wire settings.