BoilerFaultCodesWorcester Bosch Flash once per second Fault Code
Also written as FLASHONCEPERSECOND — same fault.
The Worcester Bosch Flash once per second fault code means: your boiler has stopped working because it has failed to light or has detected a dangerous build-up of heat.. It appears on 1 Worcester Bosch model in our database.
Technical description: Ignition lockout, Overheat lockout, Internal error (reset lockout by gently pressing reset button for 5 seconds). Common causes: No gas or low gas pressure, Weak spark or spark not across elecrodes, Gas valve leaking or failed, Low water pressure or boiler dry, Pump fail, Primary sensor position, Flame sensor fail or connector, Overheat stat tripped or disconnected or faulty, Blocked burner, Blocked heat exchanger, Faulty harness, Board fault.
What causes the Worcester Bosch Flash once per second fault?
This fault occurs when the boiler tries to light but fails to detect a flame, or if the internal temperature becomes unsafely high. It is often caused by a lack of fuel reaching the burner, a worn-out ignition spark, or a pump failure that prevents water from moving, leading to immediate overheating.
This issue often surfaces during the first cold snap of autumn when the pump or gas valve seizes up after being inactive over the summer.
DIY fix possible
Check that your gas supply is turned on (e.g., check other gas appliances)Check the pressure gauge is between 1.0 and 1.5 barPress and hold the reset button for five seconds
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Gas Safe Register →Worcester Bosch models that show the Flash once per second fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch Flash once per second 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