Boiler displays A11 and will not ignite; fan may run briefly

Safety: Turn off gas and power before removing the burner cover. Risk of gas leak and electric shock.

• 1. Reset once: Attempt a single reset. If A11 returns, do not continue resetting without inspection.
• 2. Inspect ionisation electrode: Remove the burner cover according to the Buderus service manual. Locate the ionisation electrode near the burner flame area. Check for soot, oxidation, or cracks in the ceramic.
• 3. Clean electrode: Gently clean the metal rod with fine emery cloth. Do not damage the ceramic insulator. Ensure the electrode tip is correctly positioned in the flame path as per Buderus specifications.
• 4. Check cable and connections: Inspect the high-voltage/ionisation cable for burns, cracks, or loose terminals at both the electrode and control board.
• 5. Verify earth connection: Ensure the boiler chassis is properly earthed and that the burner assembly has a solid metal contact to the chassis.
• 6. Gas and combustion check: Poor combustion can cause unstable flame detection. A qualified technician should check gas pressure, injector, and perform a flue gas analysis if the problem persists.

Note: If you are not trained to work on gas components, limit yourself to visual checks and call a certified Buderus technician.