Gen 3 thermostat showing e74 code.

[ttefft]

Have an older Carrier air handler and Payne heat pump which replaced original Carrier Heat pump.  Air handler supplies supplemental and/or emergency heat.  Thermostat is 3(?) years old and has worked pretty well. Originally used Rh and Rc connection per Nest.  18  months or so ago heat pump was not coming on.  Called Tech and after checking system he moved Rc wire to Oc and said he was reprograming and he found this to be a common problem.  System worked well until now.  I checked outdoor fuses on Heat Pump and they are Ok.  Checked all 3 Breakers on system.  Please provide any guidance you can.  Thermostat will charge with a usb connection but then states it must be remounted to complete battery charging.  Reconnect and was told all updates were complete and the thermosta  t must shut down to complete battery charge.  Thermostat goes through shut down, green light goes out and then nothing.

[David_K · Accepted solution]

The E74 error code means no power to the Rh wire is being detected. Here's some things to check:

1. Check the system power

First, make sure the power to your system is on. The power switch will usually be in your breaker box or fuse box, but in some homes it will look like a light switch near the heating or cooling equipment. 

1. There may be a single switch, or two switches: one for heating and one for cooling. Make sure the power is on for both then check your thermostat.
2. After you check the power, go to your thermostat and check if it’s working.

2. Check the thermostat wires

Important: Protect yourself and your thermostat and your system. Turn off power to your system and thermostat at the breaker or fuse box before exposing or adjusting any wiring.

1. Pull off the thermostat display.
2. Check that the wires are inserted correctly in the thermostat.

• Check that each wire is fully inserted into its connector and that every connector with a wire is down.
• Check that each wire has ⅜ inch (6mm) of exposed wire.
• Turn the power to your system back on and check to see if your thermostat is working.
• Check that each wire is connected properly to the system board.

If you’ve completed all the troubleshooting steps above and your thermostat still isn’t working correctly, we'd recommend contacting a local Nest pro for more help.

[Jake]

Hey ttefft,

I wanted to follow up and ensure you are good to go. Please let me know if you are still having any trouble from here, as I would be happy to take a closer look and assist you further. 

Thanks so much davidking for the helpful post. 

Best regards,
Jake

[Jake]

Hey ttefft,

I want to ensure you are good to go, and everything is working properly. Please let me know if you are still having trouble, as I will be locking the thread in 24 hours due to inactivity. 

Best regards,
Jake

[Jake]

Hey ttefft,

It has been a few days since the last reply, and I am going to lock the thread at this time. If you have any questions, please feel free to create a new post. 

Best regards,
Jake