Heat icon flashes but furnace does not start

Safety first: Turn off power to the furnace at the breaker before working on wiring. Be aware of sharp sheet metal edges inside the furnace cabinet.

Steps to troubleshoot:

• Check thermostat wiring: Remove the thermostat and verify the W (or W1) wire is fully inserted and clamped in the W terminal. Tug gently to ensure it is secure.
• Inspect furnace control board: With power off, remove the blower door and confirm the W wire from the thermostat is securely attached to the W terminal on the board.
• Look for error lights on furnace: Restore power and observe the furnace control board LED. If it is flashing an error code, consult the furnace manual; the furnace may be in lockout due to a separate issue (e.g., flame failure).
• Bypass test (advanced): With power on and the furnace door switch engaged, briefly jumper R to W at the furnace control board. If the furnace starts, the issue is between the thermostat and furnace (wiring or thermostat output).

When to replace: If wiring is intact and the furnace responds to a manual R‑W jumper but not to the thermostat, the W output relay in the thermostat may be defective and the thermostat should be replaced.