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

Safety first: Turn off power to the furnace/air handler and outdoor unit at the breakers before opening any panels.

• Step 1 – Check breaker and service switch: Verify that the furnace/air handler breaker is on and that any service switch near the unit is in the ON position.
• Step 2 – Inspect thermostat wiring: Remove the Geo thermostat and confirm that the R and C wires are firmly connected to their terminals on the wall plate.
• Step 3 – Measure voltage at thermostat: With power restored and using a multimeter set to AC volts, measure between R and C at the wall plate. You should see ~24 VAC. If not, proceed to the furnace.
• Step 4 – Check furnace control board fuse: Remove the furnace/air handler access panel and locate the low‑voltage control board. Inspect the small blade fuse (typically 3–5 A). Replace if blown, but only after checking for shorts in thermostat wiring.
• Step 5 – Measure voltage at control board: Measure between R and C at the control board. If no voltage is present, the 24 VAC transformer may be failed and should be replaced by a qualified technician.
• Step 6 – Replace thermostat if powered but blank: If 24 VAC is present at the wall plate but the Geo thermostat remains blank, the thermostat head unit is likely defective and should be replaced.