Fast flash High

Worcester 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 19 Worcester Bosch models in our database.

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

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.

Typical repair cost £120-350 (including labour)

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.

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.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026