Thermostat display completely dead, no backlight, and HVAC does not respond.

Safety first: Turn off mains power to the HVAC system at the breaker before opening any panels.

• Step 1 – Check other equipment: See if the furnace/air handler has any LEDs or indicators lit. If everything is off, suspect a tripped breaker or blown transformer fuse.
• Step 2 – Inspect thermostat mounting: Ensure the thermostat is firmly seated on its Atlantic wall base. Reseat it by removing and snapping it back into place.
• Step 3 – Check low‑voltage fuse: Open the HVAC unit’s control compartment and locate the low‑voltage fuse (often 3–5 A automotive‑style). Replace if blown, but only after checking for shorts in thermostat wiring.
• Step 4 – Measure transformer output: With a multimeter, measure the 24 V AC output at the transformer. If there is no voltage, replace the transformer.
• Step 5 – Verify R and C wiring: At the thermostat backplate, confirm that R (or Rh/Rc) and C are connected and secure. Tighten any loose terminals.
• Step 6 – Replace thermostat: If correct voltage is present at the backplate but the thermostat remains dead, the internal power circuit is likely failed. Replace the thermostat module.