Control displays B2, furnace may not fire or may shut down on safety

Safety: Turn off power and allow the furnace and flue to cool. Exhaust components can be extremely hot.

Steps:

• Locate flue gas sensor: Typically mounted in the flue collector or exhaust outlet of the Buderus furnace.
• Inspect sensor and wiring: Look for melted insulation, brittle wires, or a cracked sensor body. Ensure the sensor is firmly seated in its port.
• Check for soot or blockage: Excessive soot or a blocked heat exchanger can cause high flue temperatures. Visually inspect the flue path and heat exchanger surfaces where accessible. Do not dismantle sealed combustion chambers unless qualified.
• Resistance test: With power off, disconnect the sensor and measure resistance. Compare to Buderus flue sensor specs. Replace if out of range.

If B2 persists after sensor replacement, a professional should inspect combustion, draft, and heat exchanger condition.