Thermostat displays P001 and will not power on fully or reboots repeatedly; screen may flicker or stay dark.

Safety warning: You will be working near 120/240 VAC and 24 VAC circuits. Turn off power to the furnace/air‑handler and AC condenser at the breakers before opening any HVAC panels. Verify power is off with a non‑contact voltage tester.

• Step 1 – Inspect thermostat wiring: Remove GLAS from the wall plate. Verify that the R and C terminals are both wired and that the wires are firmly clamped with no copper exposed or broken.
• Step 2 – Check for C‑wire presence: GLAS requires a dedicated C‑wire for reliable power. If no wire is connected to C, check behind the wall for an unused conductor that can be connected to C at the furnace control board.
• Step 3 – Inspect HVAC control board fuse: With power off, remove the furnace/air‑handler access panel. Locate the low‑voltage 3–5 A automotive‑style fuse on the control board. If blown, replace with the same rating and inspect for shorts in thermostat wiring.
• Step 4 – Measure 24 VAC: With power restored, use a multimeter set to AC volts to measure between R and C at the furnace board and at the GLAS wall plate. You should see ~24 VAC. If present at the board but not at the wall, suspect damaged cable.
• Step 5 – Check for shorts: Inspect thermostat cable along its run (if accessible) for staples through the jacket or crushed sections. At both ends, ensure no strands are touching adjacent terminals.
• Step 6 – Replace transformer or board: If no 24 VAC is present at the control board, the furnace transformer or control board may be failed. This repair should be done by a qualified HVAC technician.