Water heater does not ignite; display shows A2 shortly after a hot water tap is opened.

Safety first: Turn the appliance OFF, close the gas shutoff valve, and disconnect power before removing covers. If you ever smell gas, ventilate the area and contact a professional immediately.

• Check gas supply: Verify the main gas cock to the Junkers heater is fully open. Confirm other gas appliances in the home work normally.
• Inspect burner area: Remove the front cover according to the user/installation manual. Look at the main burner and combustion chamber for heavy dust, rust flakes, or spider webs. Gently vacuum with a soft brush; do not bend burner fins.
• Clean ignition electrode: Locate the ignition electrode near the burner. Carefully clean the ceramic and metal tip with fine emery cloth or a soft brush. Ensure the electrode gap and position match the service manual (do not bend excessively).
• Check ionization (flame) sensor: On many Junkers models the ignition and ionization are combined in one rod. Clean it as above and ensure the wire connection is tight and not corroded.
• Reset the heater: Restore gas and power, then press the reset button or cycle power as described in the manual. Open a hot water tap and observe ignition. If A2 reappears, stop and call a qualified Junkers/Bosch service technician.