Boiler locks out, burner does not ignite, red fault LED on, display shows A01

Safety first: Turn off the boiler at the main switch and close the gas isolation valve before removing any covers. Allow the boiler to cool fully.

• Reset the boiler: Press the reset button as per the Herz manual and observe if the boiler attempts ignition (you should hear clicking from the ignition transformer).
• Check gas supply: Ensure the external gas cock is open and other gas appliances work. If gas supply is suspect, contact your gas provider or a licensed gas technician.
• Inspect ionisation/ignition electrode: Remove the burner cover, locate the Herz ignition/ionisation electrode assembly, and check for cracks, heavy soot, or incorrect gap. Clean gently with fine emery cloth and refit securely.
• Check burner and injector: Inspect the Herz premix burner and injector for dirt or debris. Carefully vacuum loose dust; do not damage the burner mesh.
• If lockouts persist: The ignition transformer or gas valve may be faulty. Do not attempt gas valve calibration yourself; contact an authorised Herz service technician.