Boiler displays A5; burner stops and may restart intermittently

Safety: Turn off the boiler and do not operate it if you suspect a flue blockage. Risk of carbon monoxide.

• 1. Visual flue inspection: From outside, check that the Buderus flue terminal is not blocked by debris, snow, nests, or vegetation. Ensure terminal clearances are respected.
• 2. Check condensate drain: A blocked condensate trap or line can cause flue gas recirculation and overheating. Inspect and clean the condensate trap and ensure the drain is free-flowing.
• 3. Locate flue gas sensor: On many Buderus boilers, the flue gas NTC is mounted on the flue collector or near the heat exchanger outlet. Inspect for damage or loose mounting.
• 4. Wiring and resistance test: With power off, check the sensor wiring and measure its resistance. Compare to Buderus NTC specs. Replace if out of range.
• 5. Check for incorrect flue assembly: If the boiler or flue has been recently worked on, verify that all flue joints are correctly assembled and sealed according to Buderus installation instructions.
• 6. Professional flue test: If the error persists or if you suspect poor combustion, a qualified technician should perform a flue gas analysis and full safety check.