Discover why your thermostat loses power and how to fix this persistent error code.
The E74 code indicates your Nest is failing to detect a consistent 24V signal from the red wire.
Nest thermostats draw extra current from your HVAC transformer, which can overwhelm older, legacy furnace systems.
Over time, HVAC transformers degrade due to heat, losing the voltage stability required by smart devices.
Use a multimeter to measure the R and C wire voltage at your furnace control board.
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