Thermostat displays L1 and indicates low battery or backup power fault (for models with internal battery).

Safety first: Turn off HVAC power before opening the thermostat housing.

• Check if battery is user‑replaceable: Consult the Payne smart thermostat manual. Some models have a replaceable coin cell or pack; others are sealed.
• Replace battery (if allowed): If the design permits, remove the old battery and install the exact specified replacement, observing polarity.
• Ensure stable 24VAC: Verify that the thermostat is receiving consistent 24VAC from the Payne control board so the backup battery is not being over‑used.
• Replace thermostat if sealed: If the battery is not user‑serviceable and L1 persists, replace the thermostat to avoid loss of settings during outages.