Thermostat screen is completely blank and does not power on at all.

Safety warning: Turn off power to furnace/air‑handler and AC condenser at the breakers before opening any panels or moving wires.

• Step 1 – Check other thermostats: If you have multiple zones, see if other thermostats are powered. If all are dead, suspect the transformer or main fuse.
• Step 2 – Inspect GLAS mounting: Ensure the thermostat is firmly seated on the wall plate and not crooked or loose.
• Step 3 – Measure voltage at wall plate: With power on and a multimeter set to AC volts, measure between R and C at the GLAS wall plate. You should see ~24 VAC. If not, proceed to the furnace.
• Step 4 – Check furnace fuse and transformer: With power off, inspect the low‑voltage fuse on the control board and replace if blown. Restore power and re‑measure voltage. If still no 24 VAC, the transformer or control board may be failed.
• Step 5 – Check thermostat cable: If 24 VAC is present at the furnace board but not at the wall plate, test continuity of R and C conductors and replace the cable if open.
• Step 6 – Replace GLAS: If 24 VAC is present at the wall plate and the thermostat is properly seated but remains blank, the GLAS main control board is likely failed and the thermostat should be replaced.