Water heater displays E2; burner does not ignite and unit locks out.

Safety: Turn off power and gas. Allow the combustion chamber to cool. Work in a well-ventilated area and follow gas safety regulations.

• Access burner assembly: Remove the front cover and combustion chamber cover as per the Buderus manual.
• Inspect flame sensor: Locate the ionization electrode near the burner. Check for soot, oxidation, or cracks in the ceramic insulator.
• Clean electrode: Gently clean the metal rod with fine emery cloth or a Scotch-Brite pad. Do not sand the ceramic. Wipe off debris.
• Check gap and position: Verify that the electrode tip is positioned correctly in the flame path and that the gap to the burner surface matches Buderus specifications.
• Inspect wiring and ground: Check the ignition/flame sensor cable for cracks or burns. Ensure the burner and chassis ground connections are tight and free of corrosion.
• Reassemble and test: Refit the combustion cover with its gasket, restore gas and power, and attempt ignition. Observe through the sight glass that a stable flame is present and that the E2 code does not return.

If the flame sensor and wiring are good but the fault persists, the control board may require professional diagnosis and replacement.