The Lennox air conditioner thermostat display is blank or unresponsive; the system does not run.

Safety: Turn off power to the indoor unit before accessing the control board.

• 1. Check thermostat batteries: Replace batteries if your Lennox thermostat uses them.
• 2. Inspect low-voltage fuse: Replace the 3–5A fuse on the control board if blown.
• 3. Verify R and C connections: Ensure the R and C wires are firmly connected at both the thermostat and the control board.
• 4. Check float switch: A tripped condensate float switch may cut power to R. Clear the drain and reset the switch.

Note: If the thermostat remains blank with proper wiring and fuse, it may need replacement.