BoilerFaultCodesWorcester Bosch 1021 Fault Code
The Worcester Bosch 1021 fault code means: your boiler is struggling to accurately measure the temperature of your hot water, which usually means your taps aren't getting hot enough or stay cold.. It appears on 5 Worcester Bosch models in our database.
Technical description: Cylinder charging or hot water temperature sensor faulty
What causes the Worcester Bosch 1021 fault?
This fault occurs when the boiler's internal computer stops receiving a reliable signal from the sensors that monitor your hot water temperature. The most common reason is that the sensor has either worn out internally or a connection in the wiring has become loose or corroded, preventing the boiler from knowing when to fire up to heat your water.
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 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch 1021 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 1000 manual (PDF)
- Worcester Bosch Greenstar 2000 manual (PDF)
- Worcester Bosch Greenstar 27CDi manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026