Hello @NestTroubleBob, Thanks for reaching out. Unlike other wires
connected to your thermostat, a C wire doesn’t control heating or
cooling functions. It only provides constant power to the Nest
thermostat. Please use the following compatibility che...
Hello @K2reasoner, Thanks for reaching out. E9 error code means that the
thermostat detects a wire in the C terminal but can't complete a
power-sharing circuit. Let's check this further. A few questions: what
type of Nest Thermostat do you have? How'...