Boiler locks out with A3; fan runs briefly then stops, burner does not ignite

Safety: Switch off power to the boiler. Do not operate the boiler if you suspect flue blockage or flue gas leakage.

• Visual flue check: From outside, inspect the Junkers concentric flue terminal for obstructions (ice, nests, leaves). Clear any visible blockage without damaging the terminal.
• Reset and observe: Restore power, reset the boiler, and observe whether the error reappears quickly. If it does, turn the boiler off again.
• Check system pressure and circulation: Ensure the heating system pressure is within the green zone (typically 1.0–1.5 bar when cold). Bleed radiators and verify that isolation valves under the boiler are fully open.
• Professional steps:
  • Access the flue gas NTC sensor on the flue hood or outlet; check resistance against Junkers specifications at room temperature.
  • Inspect the wiring harness and connector for corrosion or loose pins; repair as needed.
  • Clean the primary heat exchanger if heavily scaled or sooted, which can cause local overheating.
  • Verify correct pump operation and that bypass/three-way valve is not stuck.