F.27 error, boiler attempts to ignite but locks out, sometimes with smell of gas at flue

Safety: Turn off gas and power immediately if you suspect gas leakage. Ventilate the area and do not use open flames.

Checks (qualified personnel recommended):

• Inspect the ionisation electrode for cracks, soot, or incorrect position relative to the burner; clean or replace as needed.
• Check the electrode cable and connection to the PCB for damage or tracking.
• Test the gas valve for tightness when closed; replace if it allows gas flow when de-energized.
• Verify correct earthing of the boiler, as poor earth can affect flame detection.

Note: Persistent F.27 requires professional diagnosis due to the risk of unsafe combustion.