Boiler displays FL or Flame Loss and shuts down during or shortly after ignition.

Safety first: Flame-related faults involve gas and combustion. Turn off power and close the gas shutoff valve before removing burner or ignition components. If you smell gas, ventilate the area and contact your gas supplier or emergency services.

• Inspect the flame sensor: Locate the flame rod on the Laars burner assembly (often a small metal rod with a ceramic insulator). Remove it carefully and clean the metal rod with fine steel wool or a Scotch-Brite pad until shiny. Do not sand the ceramic. Reinstall and ensure the wiring connection is tight.
• Check burner ground: Verify that the burner assembly and control board ground wires are securely attached to clean metal surfaces. Loose or corroded grounds can cause false flame loss.
• Observe ignition: With the front panel in place (or using the viewing port), restore power and call for heat. Watch through the sight glass: does the burner light and then go out after a few seconds? If so, the flame sensor or gas supply may be at fault.
• Check gas supply basics: Ensure the manual gas shutoff valve to the Laars boiler is fully open. If other gas appliances are also having issues, contact your gas supplier.

When to call a professional: If cleaning the flame sensor does not resolve FL, or if the burner fails to light consistently, a technician should check gas pressure, combustion, and the ignition control circuitry.