8/1/1 Medium

Worcester Bosch 8/1/1 Fault Code

Also written as 811 — same fault.

The Worcester Bosch 8/1/1 fault code means: your boiler is struggling to complete a routine safety cycle intended to kill bacteria in the hot water system, likely because it cannot properly regulate the temperature or flow of the water.. It appears on 1 Worcester Bosch model in our database.

Book a service

This fault should be looked at by a Gas Safe engineer within the next week or two. It may get worse if left.

Technical description: Thermal disinfection has failed. Outlet flow during disinfection period too high, sensor lead broken or shorted, sensor connected incorrectly or defective, filling pump defective.
Typical repair cost £150-300 (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 8/1/1 fault code

Click a model for the full fault code list and model-specific repair notes.

Source

Worcester Bosch 8/1/1 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 7 July 2026