BoilerFaultCodesWorcester Bosch D1 Fault Code
The Worcester Bosch D1 fault code means: your boiler is struggling to detect the speed at which heating water is returning to it, which will prevent your heating and hot water from working properly.. It appears on 9 Worcester Bosch models in our database.
Technical description: Return water flow speed regulator
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 D1 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch D1 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 12i System manual (PDF)
- Worcester Bosch Greenstar 25Si Compact ErP manual (PDF)
- Worcester Bosch Greenstar CDi Compact manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 31 August 2026