BoilerFaultCodesWorcester Bosch A8 Fault Code
The Worcester Bosch A8 fault code means: your boiler's internal components are having trouble communicating with the main control board, preventing the system from starting up.. It appears on 28 Worcester Bosch models in our database.
Technical description: DHW sensor fault: DHW sensor (8000 - 20000 ohms at room temperature), Wiring and connections to sensor.
What causes the Worcester Bosch A8 fault?
This fault occurs when the boiler's main control board loses its digital connection with internal components like the display or the heat engine sensors. It is usually caused by a loose connection in the internal wiring or a physical failure of the electronic circuit board itself, preventing the boiler from sending start-up commands. In some cases, moisture ingress or a power surge may have damaged the sensitive communication pins on the board.
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 A8 fault code
Click a model for the full fault code list and model-specific repair notes.
- 35 RSF Repair: £90-180
- 537i Repair: £150-250
- CDi Classic Repair: £90-£250
- CDi Highflow Repair: £150-350
- Greenstar 12i System Repair: £180-450
- Greenstar 25 HE Combi Repair: £180-450
- Greenstar 25HE Repair: £150-450
- Greenstar 28CDi Repair: £150-300
- Greenstar 30CDi Classic System ErP Repair: £0-180
- Greenstar 30HE Repair: £150-£450
- Greenstar 30i Repair: £90-250
- Greenstar 30si Repair: £120-250
- Greenstar CDi Compact Repair: £60-220
- Greenstar i Repair: £180-450
- Greenstar i Junior Repair: £120-220
- Greenstar R29 40 HE Repair: £150-300
- Greenstar Ri Repair: £180-450
- Greenstar Si Combi / i System Repair: £0-180
- Greenstar System Repair: £150-300
- Heatslave Repair: £90-180
- Hideaway 90 Repair: £90-220
- Highflow 440 CDi Repair: £200-450
- Highflow 550 CDi Repair: £200-450
- Highflow CDi Repair: £150-300
- Junior 28i Repair: £90-180
- R30 HE Plus Repair: £150-350
- Worcester 25/30 HE Repair: £180-450
- ZWB 7-25 Repair: £150-350
Source
Worcester Bosch A8 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 35 RSF manual (PDF)
- Worcester Bosch 537i manual (PDF)
- Worcester Bosch CDi Classic manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026