The thermostat screen is completely blank and does not power on.

Safety warning: Turn off the power to the 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 unresponsive, suspect the transformer or the main fuse.
• Step 2 – Inspect GLAS mounting: Ensure that the thermostat is firmly seated on the wall plate and not crooked or loose.
• Step 3 – Measure voltage at wall plate: With the power on and a multimeter set to AC volts, measure the voltage 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 the power off, inspect the low-voltage fuse on the control board and replace it if blown. Restore power and re-measure the voltage. If there is still no 24 VAC, the transformer or the control board may have failed.
• Step 5 – Check thermostat cable: If 24 VAC is present at the furnace board but not at the wall plate, test the continuity of the R and C conductors and replace the cable if it is 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 has likely failed and the thermostat should be replaced.