Furnace locks out with error 103, fan runs but no ignition sequence starts.

Safety: Disconnect power and close the gas valve before touching ignition components.

• Visual inspection: Remove the burner access panel. Locate the ignition electrode (often combined with the flame sensor on Ariston units). Check for cracked ceramic, broken tip, or burnt marks.
• Clean and adjust electrode: Gently clean the electrode tip with fine abrasive pad. Check the gap between the electrode and burner surface; adjust to the specification in the Ariston manual (commonly around 3–4 mm) using gentle pressure.
• Inspect ignition cable: Follow the high-voltage cable from the electrode to the control board or ignition transformer. Look for cuts, burn marks, or loose connectors. Reseat both ends firmly.
• Check for moisture: Condensation or water ingress in the burner compartment can cause ignition failure. Dry any visible moisture and ensure the flue and condensate drain are not leaking into the chamber.
• Reset and test: Restore power and gas, reset the furnace, and call for heat. Listen for the clicking of the igniter. If there is no spark sound and 103 returns, the ignition transformer or control board likely needs replacement by a professional.