104
Ariston
Boiler Lockout Detected by Thermostat
Error 104 displayed; thermostat reports that the boiler is in lockout and cannot start heating.
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Safety Warning
This repair may involve working with high voltage components or water connections. Always unplug the appliance before removing any panels.
If you are not confident in your ability to perform this repair safely, we strongly recommend contacting a professional technician.
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Possible Causes
Boiler ignition failure, Flame detection fault, Low system water pressure, Overheat thermostat tripped on boiler
How to Fix / Troubleshooting
Safety: Do not open the boiler combustion chamber. Work only on user-accessible controls.
- Check boiler display: Look at the boiler’s own screen or LEDs for a specific fault code (e.g. ignition failure, low pressure). Follow the boiler user manual instructions to reset.
- Verify system pressure: On the boiler pressure gauge, ensure pressure is within the recommended range (typically 1.0–1.5 bar when cold). If low, top up via the filling loop as per the boiler manual.
- Reset boiler: Use the boiler’s reset button or dial. After reset, wait a few minutes to see if the thermostat clears error 104.
- Bleed radiators: If air is trapped, bleed radiators starting from the highest floor, then recheck pressure.
- Persistent lockout: If the boiler repeatedly locks out, a gas, ignition, or overheat issue exists. Contact a Gas Safe/qualified technician; do not attempt internal boiler repairs.
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Repair Difficulty
Professional Required
5/5
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