The furnace does not start at all; there is no inducer motor operation, no ignitor glow, and no blower activity. The LED flashes code 11 (no previous code stored).

Safety first: Turn off the power to the furnace at the service switch and circuit breaker before touching any wiring or components.

• Verify power supply: Confirm the furnace service switch is in the ON position and the circuit breaker is not tripped. Reset the circuit breaker once if needed.
• Check door switch: Ensure the blower compartment door is fully closed and engaging the door safety switch on the control board bracket.
• Inspect thermostat wiring: Remove the furnace front panel, locate the low-voltage terminal strip on the control board (R, C, W, Y, G). Ensure the W (heat call) and R wires are firmly attached and not broken.
• Call for heat test: With the power OFF, temporarily jumper R to W at the control board. Restore the power and see if the furnace begins a heat cycle. If it does, the issue may be with the thermostat or thermostat cable.
• Clear code: Turn the power OFF for 30 seconds, then ON. If code 11 returns but the furnace operates normally, it is only indicating no stored fault.

If the furnace still does not respond to a heat call and only shows code 11, the control board may not be processing the call and should be professionally tested.