Furnace locks out shortly after start, burner does not ignite, display shows A3

Safety first: Turn off the main gas supply and isolate electrical power to the furnace before removing any covers. Wait for all components to cool.

• 1. Reset and observe: Restore power, reset the furnace according to the Junkers manual (usually by pressing the reset button), and watch the ignition sequence through the inspection window. Check if you see sparking and if the burner lights briefly or not at all.
• 2. Inspect ionisation electrode: Turn power and gas back off. Remove the burner cover. Locate the ionisation electrode (flame sensor) near the burner. Check for soot, oxidation, or cracks. Clean the metal rod gently with fine emery cloth or a non-abrasive scouring pad. Ensure the ceramic insulator is intact and not cracked.
• 3. Check electrode position: Verify the ionisation electrode tip is correctly positioned in the flame path as specified in the Junkers service manual (usually a few millimetres above the burner surface). Adjust carefully if bent.
• 4. Inspect ignition electrode and cable: Check the ignition electrode for cracks and carbon tracking. Inspect the high-voltage cable for burns, cuts, or loose connections at the ignition transformer and electrode. Reseat connectors firmly.
• 5. Burner and injector check: Remove the burner assembly if accessible. Clean burner ports with a soft brush and compressed air. Do not enlarge injector orifices. If injectors are visibly blocked or corroded, they should be replaced.
• 6. Gas valve operation: With a manometer and according to Junkers specifications, a qualified technician should verify that the gas valve opens and that inlet and outlet pressures are within range. If the valve does not open or coil resistance is out of spec, the gas valve assembly must be replaced.
• 7. When to call a professional: If cleaning and repositioning the electrodes does not restore reliable ignition, or if you suspect gas valve or control board failure, contact an authorised Junkers service technician.