Thermostat shows W001 and will not activate the heating system when heat is demanded.

Safety warning: Turn off power to the furnace/air‑handler at the breaker before touching any wiring inside the HVAC unit.

• Step 1 – Inspect GLAS W terminal: Remove the thermostat and verify the W (or W1) wire is fully inserted and clamped. Tug gently to ensure it is secure.
• Step 2 – Inspect furnace control board: With power off, remove the furnace panel and locate the thermostat terminals. Confirm the W wire is connected to the W terminal and firmly tightened.
• Step 3 – Continuity test: With power off, disconnect the W wire at both ends and use a multimeter to check continuity of the W conductor. Replace the thermostat cable if open.
• Step 4 – Bypass test (advanced): With power on and caution, briefly jumper R to W at the furnace control board. If the furnace starts heating, the HVAC system is functional and the issue is between GLAS and the board.
• Step 5 – Suspect GLAS relay: If wiring is intact and the furnace responds to an R‑W jumper but not to GLAS, the internal W relay on the GLAS main board may be failed. Replace the thermostat.