BoilerFaultCodesWorcester Bosch 1021 B Fault Code
Also written as 1021B — same fault.
The Worcester Bosch 1021 B fault code means: your boiler is unable to provide hot water because it cannot accurately measure how hot the water is.. It appears on 4 Worcester Bosch models in our database.
Technical description: Hot water temperature sensor is defective – the plug to the hot water temperature sensor may not be connected
What causes the Worcester Bosch 1021 B fault?
This fault occurs when the boiler's control board loses communication with the sensor that monitors your tap water temperature. The most common reason is that the sensor itself has developed an internal electrical failure or the wiring connecting it to the main system has come loose or become corroded. Because the boiler cannot safely verify how hot the water is, it stops heating it entirely to prevent the risk of scalding.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Worcester Bosch models that show the 1021 B fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch 1021 B fault code information on this page is sourced and cross-checked against the official Worcester Bosch service / installation manuals for each affected model.
- Worcester Bosch Greenstar 2000 manual (PDF)
- Worcester Bosch Greenstar 8000 Life & Style manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026