Furnace locks out with 3L code, burner starts briefly then shuts down

Safety: Switch off power and close the gas shutoff valve before opening the burner compartment.

Inspection steps:

• Check gas supply stability: Reopen the gas valve and observe other gas appliances while the furnace runs. If they flicker or drop out, there may be a supply or regulator issue that requires a gas utility or HVAC professional.
• Clean flame sensor: Remove the flame rod from the Buderus burner. Gently clean it with fine steel wool or emery cloth until the metal is bright. Avoid touching the cleaned surface with bare fingers. Reinstall and ensure the mounting bracket is tight for good grounding.
• Inspect burner head: Remove the burner assembly if accessible. Check for rust, debris, or soot blocking the burner ports. Clean with a soft brush and vacuum. Do not enlarge burner orifices.
• Check combustion air: Ensure the combustion air intake (especially on sealed-combustion Buderus condensing furnaces) is not blocked by snow, leaves, or debris. Clear both intake and exhaust terminations.
• Reset and test: Restore power and gas, reset the control, and call for heat. Watch the flame through the sight glass. If the flame is lifting, yellow, or unstable, shut down and call a technician for combustion analysis.

Professional help: Persistent 3L codes often indicate a failing burner control module or gas valve, which should be diagnosed and replaced by a Buderus-trained technician.