Thermostat shows L2 and indicates a C‑wire or common power fault; device may be running on battery only (if equipped) or may reboot frequently.

Safety first: Turn off power to the furnace/air handler before working on low‑voltage wiring.

• Verify C at control board: Open the furnace/air handler control compartment and locate the C terminal on the Payne control board. Confirm a wire is connected and that it goes to the thermostat location.
• Check C at thermostat: Remove the thermostat and ensure the same conductor is landed on the C terminal. Tighten the screw and ensure good copper contact.
• Test continuity (advanced): With power off, use a multimeter to check continuity of the C conductor between the thermostat and control board. Repair or replace the cable if open.
• Add a C wire (if missing): If your Payne system does not have a spare conductor, consider running a new cable or using a manufacturer‑approved C‑wire adapter module.
• Correct external transformer wiring: If an external 24V adapter is used, ensure its common is properly tied to the Payne system common per the adapter instructions.