123 Medium

Worcester Bosch 123 Fault Code

The Worcester Bosch 123 fault code means: your boiler is confused because it is receiving an incorrect signal from a temperature sensor, which is preventing it from heating your water or home properly.. 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: Cylinder temperature sensor type used as collector temperature sensor (T1).
Typical repair cost £120-200 (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 123 fault code

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

Source

Worcester Bosch 123 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