BoilerFaultCodesWorcester Bosch Fast flash Fault Code
Also written as FASTFLASH — same fault.
The Worcester Bosch Fast flash fault code means: your boiler has detected a critical internal fault and has locked itself out to prevent damage or unsafe operation.. It appears on 12 Worcester Bosch models in our database.
Technical description: Volatile lockout (sensor, fan or code plug): Temperature sensor (8000 - 20000Ω); Wiring/connections to sensor; Fan/cable/connections; Code plug fitted.
What causes the Worcester Bosch Fast flash fault?
This fault occurs when the boiler's internal computer detects a conflict or failure in its safety sensors or the fan that clears exhaust gases. The most common reason is that a sensor has stopped reporting the correct temperature or the fan is no longer spinning at the required speed, causing the system to shut down for safety. Since the boiler cannot confirm the system is venting or cooling correctly, it locks itself out to prevent overheating.
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 Fast flash fault code
Click a model for the full fault code list and model-specific repair notes.
- 24CDi BF Repair: £150-450
- 26 CDi Xtra Repair: £180-450
- 28 i SI Repair: £150-300
- 28 Si RSF Repair: £150-300
- 28i Junior Repair: £150-350
- Greenstar 12Ri Repair: £120-350
- Greenstar 24i Junior Repair: £120-250
- Greenstar 25Si Repair: £150-300
- Greenstar 30i Repair: £150-£450
- Greenstar i Junior Repair: £150-350
- Greenstar Si Repair: £80-250
- Hideaway 90 Repair: £180-450
Source
Worcester Bosch Fast flash 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 24CDi BF manual (PDF)
- Worcester Bosch 26 CDi Xtra manual (PDF)
- Worcester Bosch 28 i SI manual (PDF)
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 7 July 2026