BoilerFaultCodesWorcester Bosch Slow flash Fault Code
Also written as SLOWFLASH — same fault.
The Worcester Bosch Slow flash fault code means: your boiler has stopped running for safety reasons and needs to be manually restarted to try and get your heating and hot water back on.. It appears on 6 Worcester Bosch models in our database.
Technical description: Burner Lockout (identified by flashing lights on facia). Can only be cleared by using the facia reset button (press for at least one second).
What causes the Worcester Bosch Slow flash fault?
This fault occurs when the boiler attempts to light the burner but fails to detect a stable flame. The most common reasons include a lack of gas reaching the unit, worn-out spark components that cannot ignite the fuel, or a sensor that is too dirty to recognize that a flame is actually present.
This issue often surfaces during the first cold snap of autumn when the boiler components are put under sudden strain after being inactive throughout the summer.
DIY fix possible
Locate the reset button on the front control panel.Press and hold the reset button for at least one second.Wait for the boiler to restart; if the light stops flashing, the boiler should resume normal operation.
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Gas Safe Register →Worcester Bosch models that show the Slow flash fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch Slow flash 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