BoilerFaultCodesWorcester Bosch H8 Fault Code
The Worcester Bosch H8 fault code means: your boiler is sensing a flame even though the gas should be turned off, leading the system to shut down as a safety precaution.. It appears on 5 Worcester Bosch models in our database.
Technical description: Flame detected after gas. Gas valve switched off. Cables and connections to gas valve. Condensate trap. Flue (for blockages). Otherwise replace control board.
What causes the Worcester Bosch H8 fault?
This fault occurs when the boiler's internal sensors continue to detect a flame or electrical current even after the gas valve has been instructed to close. The most common reason is a faulty gas valve that is failing to shut off properly or a 'ghost' signal caused by a malfunctioning control board or dampness around the electrodes. Because the system thinks a fire is still burning when it shouldn't be, it shuts down immediately to ensure your safety.
This fault can occasionally be triggered during periods of heavy rain or high humidity if moisture causes a short circuit in the electrical connections or blockages in the flue.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Worcester Bosch models that show the H8 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch H8 fault code information on this page is sourced and cross-checked against the official Worcester Bosch service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026