Thermostat reports 'No C wire' or requires constant battery replacement

Safety first: Turn off power to the furnace/air handler at the breaker before modifying any wiring.

Steps to troubleshoot:

• 1. Check thermostat terminals: Remove the thermostat and look for a wire connected to the C terminal. If none is present, your system may not have a dedicated common wire.
• 2. Inspect furnace control board: Remove the furnace access panel and locate the low-voltage terminal strip (R, C, W, Y, G, etc.). Look for an unused conductor from the thermostat cable that can be connected to C.
• 3. Connect spare conductor: If a spare wire is available, connect it to the C terminal at the furnace and to the C terminal at the thermostat.
• 4. Use a Resideo C-wire adapter: If no spare conductor exists, install a Resideo C-wire adapter kit according to the included wiring diagram.
• 5. Verify operation: Restore power and confirm the thermostat powers up without relying on batteries and no longer reports C-wire issues.

When to call a professional: If you are unsure which wires to move or your system uses a boiler or heat pump with complex wiring, have an HVAC technician install the C wire or adapter.