BoilerFaultCodesWorcester Bosch 1.b Fault Code
Also written as 1B — same fault.
The Worcester Bosch 1.b fault code means: your boiler is currently running at its maximum power level to provide hot water and this code is often just an operational status indicator rather than a breakdown.. It appears on 1 Worcester Bosch model in our database.
Technical description: Max. output (hotwater - combi only)
What causes the Worcester Bosch 1.b fault?
This code appears when the boiler detects a high demand for hot water, causing it to ramp up to its full heating capacity. It is physically triggered by the internal flow sensor detecting water movement, which tells the burner to provide maximum heat to your taps. While usually just a status update, if it stays on without you running a tap, it could mean a component like the diverter valve is stuck or leaking.
This code is slightly more frequent in winter when incoming mains water is much colder, forcing the boiler to work at maximum power for longer to reach the desired temperature.
DIY fix possible
Check if any hot water taps are fully turned on and close themPress the Reset button once to clear the display if the code persistsMonitor the boiler to see if the code disappears after hot water use ends
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Gas Safe Register →Worcester Bosch models that show the 1.b fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch 1.b 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