BoilerFaultCodesWorcester Bosch Fast flash / Off Fault Code
Also written as FASTFLASHOFF — same fault.
The Worcester Bosch Fast flash / Off fault code means: your boiler has shut itself down for safety because it has detected a significant electrical or mechanical issue with its internal components.. It appears on 4 Worcester Bosch models in our database.
Technical description: Volatile lockout - Check Temperature sensor (8000-20,000Ω), Low voltage wiring harness / connections to sensor, Fan / fan harness / connections to fan, Code plug fitted.
What causes the Worcester Bosch Fast flash / Off fault?
This fault occurs when the boiler's internal computer, the Printed Circuit Board, suffers a critical failure or receives abnormal electrical signals it cannot process. The most common reasons are a malfunctioning circuit board, a faulty ignition system that is confusing the controls, or a sudden electrical surge that has damaged the sensitive internal components.
DIY fix possible
Press and hold the 'Reset' button for five secondsWait for the indicator light to stop flashing or change to a steady lightIf the boiler refires, check that your thermostat is calling for heat
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Gas Safe Register →Worcester Bosch models that show the Fast flash / Off fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch Fast flash / Off 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