Unit attempts to start but shuts down with A3; fan may run briefly, burner does not stay lit.

Safety: Switch off the heater and allow it to cool completely. Do not operate the unit with a suspected flue blockage.

• Check air intake and flue terminal: Go outside and inspect the concentric flue terminal. Remove leaves, plastic bags, bird nests, or snow/ice blocking the outlet or inlet.
• Inspect flue pipe routing: Ensure the flue is not crushed, disconnected, or sagging with condensate. Any visible damage requires a professional.
• Clean around heat exchanger: With power off and cover removed, visually inspect the primary heat exchanger for heavy soot or dust. Light surface dust can be vacuumed; heavy soot indicates combustion issues and needs a technician.
• Check sensor connection: Locate the flue gas NTC sensor on the flue hood. Ensure its plug is firmly seated and wires are not damaged or overheated.
• Reset and test: Restore power and reset the fault. If A3 returns quickly, discontinue use and contact an authorized Junkers service center.