Thermostat calls for heat but furnace does not start (no error on furnace board)

Safety: Turn off furnace power at the breaker before working on wiring.

• Verify call for heat: Confirm the thermostat display shows that heat is ON or calling.
• Check W wire at thermostat: Remove the thermostat and ensure the W wire is fully inserted and clamped in the W terminal.
• Check W wire at furnace: At the control board, verify the W wire is secure and not corroded or broken.
• Bypass test (advanced): With power off, temporarily jumper R to W at the furnace control board. Restore power; if the furnace starts, the issue is between the board and thermostat (wiring or thermostat).

If wiring is intact and the furnace responds to an R–W jumper but not to the thermostat, the thermostat’s W output is likely faulty and the thermostat should be replaced.