Nest or Transformer Bad?

[Mike/Mass]

We/ve got a 2-zone/2-furnace setup, and I recently (3 months ago) installed the latest Nest thermostats. Generally they seem to work fine for a month or so, then the 1st floor started showing codes like E73 which said there was no power to the nest. After messing around with all sorts of ideas... I decided to run a new 5-wire thermostat line from the Nest to the Furnace. Previously, both thermostats worked with a 4-wire cable but after reading I decided that having that common blue wire might help? I actually wasn't sure where to connect the blue wire on the furnace so I had the HVAC guy come out and he wired it. Everything seemed fine and for the past month no issues.

This weekend, it started again... the E73 code popped up, heat wasn't on and it was 54° on the 1st floor. I drained the condensate pump and also blew out the OUT line with my compressor. I will hit the store later to get some white vinegar and just for licks, clean the condensate pump which is maybe 2 years old. Watching some YouTube videos, I saw that the voltage in Nest battery was low (about 3.0v) so I'm charging it right now with an external charger. I also saw where you can jump the W1 wire to the Rh wire and the furnace went on instantly but of course its bypassing the thermostat altogether.

The furnace is circa 1990's and its the more efficient type with a plastic stack, not connected to the chimney. The transformer is a sort of external device and I'm wondering if it's bad? Why else would the Nest battery not be charging as needed? So maybe I'm missing something, but could something like a transformer work then fade out or go off? TIA for any advice, if I can't do a simple fix I'll call the HVAC guy back.-Mike

 

[Mike/Mass]

Finally got the HVAC guy over and after trying a bunch of things, he determined that crimp connector was too loose on one of the various internal sensors. So the fan wasn't coming on and the plenum would overheat and shut down everything including the voltage going to charge the Nest because the fan wasn't pushing the heated air up thru the ducting. Cut off the crimp, installed a new one, and BAM! the fan came on the minute heat was called for then the furnace started... the correct sequence.