BoilerFaultCodesWorcester Bosch H4 Fault Code
The Worcester Bosch H4 fault code means: your boiler is sensing a flame even though it hasn't been asked to turn on, which is often caused by moisture or a sensor fault.. It appears on 4 Worcester Bosch models in our database.
Technical description: Flame detected with no demand. Gas valve. Flame sense electrodes/cables. Moisture on control board. Blocked condensate trap. Otherwise replace control board.
What causes the Worcester Bosch H4 fault?
This fault occurs when the boiler's internal sensors detect a flame while the unit is supposed to be off, which is a significant safety conflict. The most common reason is moisture or dampness affecting the control board or electrodes, but it can also be caused by a gas valve that isn't closing fully or a blocked drainage system causing water to back up into the combustion chamber.
This issue is more frequent during periods of heavy rain or extreme cold, as external moisture or a blocked condensate pipe can cause water to build up inside the appliance.
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 H4 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch H4 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