I have had the e74 error on my upstairs nest ever since install. I have a 3rd gen learning nest. My downstairs worked perfectly fine. Both were installed within 2 weeks of each other. After troubleshooting and calling HVAC professionals it was determined it was a nest thermostat base issue. *A sidenote is that when the HVAC guys came they rewired both my upstairs and downstairs nests. They put my red wire in Rc instead of Rh temporarily. Not sure if this was to troubleshoot or to ensure I had some sort of air/power to my upstairs but this context may be important for later.
After exchanging for a new base, I noticed that only cold air would come out my upstairs vent. The red wire was in the Rh slot. I did forget to change the downstairs nest red wire back from Rc to Rh and I am not sure if that would have made a difference (downstairs was still heating). I spent days of troubleshooting (turning heat balance off, adjusting the compressor lockout, etc). I figure my furnace is fine as my downstairs heats ok. When the upstairs claims it is heating it usually states it is using aux heat but the air is cold.
As of now, I am getting the e74 error code again. Newly exchanged base. Wires are in correctly. Totally unsure what to do at this point. I am assuming I will have to call professionals back over again but at this point I will be spending more on fixing this issue than I have on the next itself. I could rewire the downstairs back to Rh of course but how would that fix the Rh error upstairs at this point?
