BoilerFaultCodesWorcester Bosch d3 Fault Code
Also written as D3 — same fault.
The Worcester Bosch d3 fault code means: your boiler has lost its connection to an essential safety internal plug, meaning it currently cannot communicate correctly with its control system.. It appears on 20 Worcester Bosch models in our database.
Technical description: external cut-off switch activated
What causes the Worcester Bosch d3 fault?
This fault occurs when the boiler's control board loses its physical connection to a specific internal safety plug that identifies how the unit should operate. The most common reason is that the plug has vibrated loose over time, sustained moisture damage, or the internal wiring harness has developed a fault. Without this connection, the boiler shuts down immediately as a safety precaution because it no longer recognizes its own configuration.
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 d3 fault code
Click a model for the full fault code list and model-specific repair notes.
- 537i Repair: £120-250
- CDi Highflow Repair: £120-250
- Greenstar 12i System Repair: £90-180
- Greenstar 2000 Repair: £100-200
- Greenstar 25 HE Combi Repair: £120-250
- Greenstar 25HE Repair: £80-180
- Greenstar 25i (ZWB 7-25) Repair: £120-220
- Greenstar 27CDi Repair: £100-200
- Greenstar 28CDi Repair: £100-200
- Greenstar 30HE Repair: £90-180
- Greenstar R29 40 HE Repair: £100-250
- Greenstar ZWB7 Repair: £100-200
- Highflow 440 CDi Repair: £150-300
- Highflow 550 CDi Repair: £150-300
- Highflow CDi Repair: £150-300
- R30 HE Plus Repair: £100-£200
- Ri 25 HE Repair: £120-200
- Worcester 25/30 HE Repair: £90-180
- ZB 7 27 HE Repair: £120-220
- ZWB 7-25 Repair: £100-200
Source
Worcester Bosch d3 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 537i manual (PDF)
- Worcester Bosch CDi Highflow manual (PDF)
- Worcester Bosch Greenstar 12i System manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026