Thermostat display is completely blank and unresponsive; HVAC system does not respond to temperature changes.

Safety: Turn off power to the furnace/air handler at the breaker before removing panels or touching wiring.

• Step 1 – Check breaker: Confirm the HVAC breaker is ON and has not tripped. Reset if necessary.
• Step 2 – Inspect furnace fuse: Remove the furnace/air handler access panel. Locate the control board and check the low‑voltage blade fuse (often 3A). Replace only with the same rating if blown. If it blows again, there is a short that needs professional diagnosis.
• Step 3 – Verify 24 VAC: With power on, measure voltage between R and C at the furnace control board. If no voltage, the transformer may be failed and should be replaced by a technician.
• Step 4 – Check thermostat wiring: At the thermostat, ensure it is firmly seated on the sub‑base and that the R and C wires are secure. Tug gently on each wire to confirm it is not broken inside the insulation.
• Step 5 – Test thermostat on known‑good system: If possible, temporarily connect the thermostat to another known‑good 24 VAC source to see if it powers up. If it remains dead, replace the thermostat.