Furnace attempts to start but shuts down quickly; LED flashes code 11 (no previous code stored).

Safety First: This code by itself is not a fault. If the furnace is otherwise operating normally, no repair is required. Always turn off power at the service switch and breaker before removing any panels.

Steps:

• Observe operation: Let the furnace run through a complete heat cycle. Confirm that the inducer motor starts, ignitor glows, burners light, and blower runs.
• Check thermostat settings: Ensure the thermostat is set to HEAT and the setpoint is above room temperature.
• Inspect power stability: If this appears after a power outage or after you turned the switch off/on, it is normal. The Comfortmaker control board (e.g., HK42FZ, HK42FZ013) simply indicates no stored fault.
• Monitor for new codes: If another code appears after a failed ignition or shutdown, troubleshoot that specific code instead.

When to call a technician: If the furnace will not heat and only shows code 11 with no other activity (no inducer, no ignitor), the control board or thermostat circuit may be at fault and should be professionally diagnosed.