Thermostat display is blank and system does not respond

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

Steps:

• 1. Check for condensate safety switch: Look for a float switch on the drain pan or in the drain line. If tripped, clear the drain and reset the switch.
• 2. Inspect low-voltage fuse: Replace the 3–5A fuse on the indoor board if blown. If it blows again, there is a short in the thermostat or condenser wiring.
• 3. Verify thermostat wiring: Remove the thermostat and check that R and C (if used) are firmly connected. Inspect the cable for damage.
• 4. Test transformer output: With power on, a qualified person can measure 24 VAC between R and C at the board. If no voltage, the transformer is likely failed.
• 5. Replace thermostat if necessary: If 24V is present at the thermostat but it remains blank, the thermostat itself may be defective.