BoilerFaultCodesWorcester Bosch 1-A3 Fault Code
Also written as 1A3 — same fault.
The Worcester Bosch 1-A3 fault code means: your boiler's heating power has been manually limited to a lower setting, which might prevent your home from reaching your desired temperature properly.. It appears on 1 Worcester Bosch model in our database.
Technical description: Maximum heating output: The maximum thermal output can be reduced via the service function 3-b1.
What causes the Worcester Bosch 1-A3 fault?
This code appears because a settings adjustment has been made to intentionally 'cap' the boiler's maximum power output. It is usually triggered when a technician or user limits the heating capacity to match a smaller property, which can sometimes result in the radiators not getting hot enough during very cold spells. It is not a component failure, but rather a notification that the boiler is not operating at its full potential wattage.
You may only notice this status during winter, as the limited power setting might struggle to keep the house warm when outside temperatures drop significantly.
DIY fix possible
Navigate to the boiler's service menu using the touchscreen or buttons.Locate 'Service Function 3-b1' in the settings.Check if the percentage value is set lower than required and adjust upward if more heat is needed.
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Gas Safe Register →Worcester Bosch models that show the 1-A3 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch 1-A3 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