Furnace will not start; status LED flashes 1 time repeatedly on Armstrong Air control board.

Safety First: Turn off electrical power to the furnace at the service switch and shut off the gas supply before removing panels. If you ever smell strong gas, evacuate and contact the gas utility or a licensed technician immediately.

Steps to check:

• Verify thermostat call for heat: Set thermostat to HEAT and raise the setpoint 5°F above room temperature. Confirm the furnace is receiving a call for heat.
• Check gas supply: Ensure the manual gas shutoff valve near the furnace is fully open (handle parallel to the pipe). Confirm other gas appliances in the home work.
• Inspect hot surface ignitor: Remove the burner access panel. Locate the hot surface ignitor on the burner assembly. Look for cracks, white spots, or breaks in the element. If damaged, replace the ignitor with an Armstrong-compatible part and avoid touching the element with bare fingers.
• Observe ignition sequence: Restore power and gas, then watch through the sight glass. The inducer should start, then the ignitor should glow bright orange. If the ignitor never glows, test for 120 VAC at the ignitor leads during ignition. If voltage is present but no glow, replace the ignitor. If no voltage, suspect the control board.
• Check gas valve operation: If the ignitor glows but burners never light, use a manometer or listen for the gas valve click. Verify 24 VAC at the gas valve terminals during ignition. If 24 VAC is present but no gas flow, replace the gas valve. If no 24 VAC, the control board or safety circuit is preventing opening.
• Reset lockout: Turn power off for 30 seconds, then back on to clear lockout. If the 1-flash code returns after several attempts, contact a qualified HVAC technician.