Furnace shows error 104 and shuts down shortly after starting, sometimes with rumbling noise.

Warning: False flame detection can prevent safe ignition. Do not bypass safety circuits.

• Power off: Turn off power at the breaker and close the gas valve.
• Inspect ionization electrode wiring: Remove the front panel and locate the flame sensor lead. Check for damaged insulation touching metal parts, which can cause a false flame signal. Reroute or replace damaged wiring.
• Clean and dry electrode: Remove the ionization electrode and ensure it is clean and completely dry. If there are signs of water tracking or corrosion, replace the electrode.
• Check grounding and neutral: Verify that the furnace is correctly wired with live, neutral, and earth as per Ariston installation instructions. Loose earth connections can cause erratic flame sensing.
• Reset and test: Restore power and gas, reset the unit, and call for heat. If error 104 persists, the control board flame detection circuit may be faulty and should be replaced by a qualified technician.