Bosch False Flame Detection / Ionisation Fault

Boiler detects flame when there should be none (false flame); may lock out.

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.

Possible Causes

Moisture or soot bridging ionisation electrode, Faulty ionisation electrode or cable, Incorrect earth or polarity on supply, PCB flame detection circuit fault

How to Fix / Troubleshooting

Safety: Isolate power and gas. Allow boiler to cool.

Steps (technician‑level):

Inspect the ionisation electrode for cracks, soot, or moisture. Clean or replace as needed.
Check the electrode lead for damage and secure connections.
Verify correct live/neutral polarity and good earth at the boiler supply.

False flame faults affect combustion safety; a qualified engineer should carry out these checks and any PCB replacement.

Repair Difficulty

Professional Required 5/5

Required Part

Ionisation Electrode

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