BoilerFaultCodesWorcester Bosch A1 Fault Code
The Worcester Bosch A1 fault code means: your boiler has detected that there is not enough water circulating through the system, which could be due to a leak or a mechanical issue with the pump.. It appears on 22 Worcester Bosch models in our database.
Technical description: Water is leaking or the pump needs replacing or freeing due to the pump running dry.
What causes the Worcester Bosch A1 fault?
This fault occurs when the boiler detects a serious lack of water pressure or circulation, often caused by a leak within the system or a failing internal pump. If the pump is spinning without enough water (running dry) or has become seized with debris, the boiler will shut down to prevent the components from overheating or burning out. In some cases, a faulty air vent can also cause an air lock that tricks the sensors into thinking the system is empty.
This issue is frequently seen in autumn when heating systems are switched back on after a long summer break, as pumps are more likely to seize if they haven't been used for several months.
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 A1 fault code
Click a model for the full fault code list and model-specific repair notes.
- 24/28/35 CDi MkII Repair: £150-300
- 28 Cdi RSF Repair: £250-450
- 35 RSF Repair: £180-450
- 532 / 537 / 542 Repair: £90-250
- 537i Repair: £100-250
- CDi Repair: £250-450
- CDi Classic Repair: £180-£350
- CDi Highflow Repair: £100-250
- CDi RSF Repair: £80-150
- Greenstar 12i System Repair: £150-300
- Greenstar 25 HE Combi Repair: £90-180
- Greenstar 25HE Repair: £100-350
- Greenstar 30i Repair: £180-350
- Greenstar CDi Compact Repair: £80-250
- Greenstar Utility 18/25 Repair: £150-300
- Heatslave Repair: £180-350
- Hideaway 90 Repair: £180-450
- Highflow 440 CDi Repair: £150-350
- Highflow 550 CDi Repair: £150-350
- Junior 28i Repair: £180-350
- R30 HE Plus Repair: £150-350
- WR325 RSF Repair: £90-160
Source
Worcester Bosch A1 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 24/28/35 CDi MkII manual (PDF)
- Worcester Bosch CDi manual (PDF)
- Worcester Bosch 35 RSF manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026