I'm doing this little write up as a suggestion to help with Nest support troubleshooting. I ran into a situation that resulted in an incorrect diagnosis of a faulty thermostat by NEST support. If I'd returned the thermostat, it would have been a waste of a perfectly good thermostat.
I was getting either an E23 or E73 error code from the NEST. This is a complaint about no voltage coming from the 24vac red wire, but when I tested with a multi-meter, it was showing 24vac. There's an additional thing you can try to narrow down what's wrong:
1) Disconnect the cooling wires (on my system Yellow) but make sure the Fan Wire (Green,) Heat Wire (White,) and 24vac wire (Red,) remain connected.
2) This will cause the NEST to switch to heat only mode
3) Now that the NEST has bypassed the circuit for cooling, it might then detect voltage on the 24vac wire, and the NEST will no longer be in error mode
4) This is an indication that the NEST is receiving power and is NOT FAULTY
5) This means the problem lies in the AC unit itself.
6) Locate the issue on the cooling call side in the AC unit. In my case, it was a tripped float switch due to a clogged drain line. Once the drain line was cleared, the circuit was remade and NEST happily worked in cooling mode again.
