The Nest Learning Thermostat can’t detect wires that are installed
The Nest Learning Thermostat detects wires by testing them in two ways: mechanically (is a wire inserted?) and electrically (is there an electrical signal on the wire?). If a required wire fails either of these tests, setup may not continue.
Here are some of the most common causes of these errors and how to fix them. For help with specific wiring errors, the troubleshooting tool widget will guide you through detailed troubleshooting steps.
Wires aren’t fully inserted
Make sure the copper part of the wire is fully inserted in each of the connectors. The ends of the wires need to be straight and they need between ⅓ and ½ inch of exposed wire for a reliable connection.
The Nest Learning Thermostat does not support stranded wires.
If your low voltage wires are stranded (each of the low voltage wires is made up of lots of smaller wires), you may run into this error. Reinstall your previous thermostat and contact a Nest Pro installer to have your thermostat wiring replaced or adapted to work with the Nest Learning Thermostat.
The Nest Learning Thermostat wire connectors require 18-22 gauge solid-core copper wires.
Wires are too small
The Nest Thermostat might not sense your wiring if the wires for your heating and cooling system are too thin. The Nest Thermostat requires wires between 22 and 18-gauge, which is standard for residential heating and cooling systems. Anything smaller than 22-gauge wire could be too thin for the Nest Thermostat to detect.
If you aren’t comfortable replacing the low voltage wiring in your system yourself, contact your local HVAC professional for help replacing these wires.