E74 error, 24V coming in, old thermostat works

[manojmmk]

Thermostat - Nest 3rd Gen Learning 

Diagnostic Update:

• Manual Thermostat Test (COMPLETED): I have successfully bypassed the Nest and installed a manual mechanical thermostat. The HVAC system (Heat and AC) operates perfectly, proving the 24V transformer and all furnace safety circuits are functional.

• Charging Test (COMPLETED): The Nest was charged via Micro-USB overnight as instructed.

• Current Status: Despite a full charge and confirmed 28V AC at the Rh wire (via multimeter), the Nest immediately returns to the E74 (No power to Rh) error when mounted to the base.

Key Data Points for Agent:

• Battery Status: The battery was fully charged overnight to confirm it wasn't the issue

• Wiring: Rh to Rc swap was previously tried and resulted in an E79 code, Removing C wire, the nest detected it was removed

• Safety Check: Furnace fuses are new and replaced as a part of diagnosis .
  
  What might be going on here? The unit was working perfectly for over 2 years. The online chat agent was not able to diagnose the issue. Sounds like the nest base plate or unit issue. The HVAC is working perfectly with old thermostat.

[tajennifer]

Hi manojmmk,

Thanks for posting. I'm sorry to hear about the situation you are experiencing with your Nest Learning Thermostat (3rd Gen), which was showing the E74 error and, after the wiring swap, is now showing the E79. I appreciate the information provided and the steps you tried to solve the situation. I understand how difficult it can be when things don't work as expected. Don't worry; help is here.

Since you are currently in contact with our support team, I recommend following up on the latest email they sent you. They will provide you with the best next steps to resolve this power issue.

I hope this information helps.

Best regards,
Jenny