Furnace will not start; control board displays E01 immediately after a call for heat.

Safety first: Turn off electrical power to the furnace at the service switch and shut off the gas supply before removing panels. Allow the furnace to cool.

Steps to try:

• Inspect and clean the flame sensor: Locate the flame sensor rod near the burner assembly (usually a thin metal rod with a single wire, mounted in front of the burner flame). Remove the mounting screw, pull out the sensor, and gently clean the rod with fine steel wool or a Scotch-Brite pad until shiny. Do not sand heavily or bend the rod. Reinstall and reconnect the wire firmly.
• Check the flame sensor harness: Follow the wire from the flame sensor back to the Alpha ignition/control board. Ensure the connector is fully seated and there are no broken or burned wires. Reseat the connector.
• Verify gas supply: Confirm the manual gas shutoff valve to the furnace is fully open. If other gas appliances are weak or not working, contact your gas supplier.
• Reset power: Restore power and gas, then call for heat at the thermostat. Observe the ignition sequence through the sight glass. If the burners light but the furnace still shows E01 and shuts down after a few seconds, the flame sensor or control board may be faulty.

When to stop: If you smell gas, hear repeated loud bangs, or the furnace locks out repeatedly, stop and contact a qualified Alpha furnace technician.