Hi wgill,
Thank you for reaching out to the community. I'm sorry that you're receiving an e74 error code on your Nest Thermostat, even though you have enough power across R and G wires. Help's here.
You get this code if your Nest Thermostat doesn't detect power to a wire or if you can't detect the wires in your Nest Thermostat.
You can try the following:
Turn off your HVAC system. The power switch will usually be in your breaker box or fuse box, but in some homes it will seem like a light switch near to the heating or cooling equipment.
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There may be a single switch or 2 switches: one for heating and one for cooling. Make sure the power is on for both, then check your thermostat.
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After you check the power, go to your thermostat and check if it's working.
After that, please check the thermostat wires. Here are the steps:
Important: Protect yourself, your Nest Thermostat, and your system. Turn off power to your system and thermostat at the breaker or fuse box before exposing or adjusting any wiring.
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Pull off the thermostat display.
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Check that the wires are inserted correctly in the thermostat.
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Check that each wire is fully inserted into its connector and that every connector with a wire is down as shown.
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Check that each wire has ⅜ inch (6 mm) of exposed wire.
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Turn the power to your system back on and check if your thermostat is working.
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Check that each wire is connected properly to the system board.
Let me know how it goes and keep me posted.
Regards,
Byron
