a/1/1 Medium

Worcester Bosch a/1/1 Fault Code

Also written as A11 — same fault.

The Worcester Bosch a/1/1 fault code means: your boiler is struggling to communicate with its wall-mounted thermostat control panel, which means it doesn't know what temperature to set.. 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: RC35-HK1 remote control. No remote control assigned, although frost protection type “FROST” has been set.
Typical repair cost £120-250 (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 a/1/1 fault code

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

Source

Worcester Bosch a/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