Fault 51 / 123 Medium

Worcester Bosch Fault 51 / 123 Fault Code

Also written as FAULT51123 — same fault.

The Worcester Bosch Fault 51 / 123 fault code means: your boiler has detected that an incorrect or incompatible temperature sensor has been installed, meaning it cannot properly monitor the system's heat levels.. It appears on 2 Worcester Bosch models 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: Incorrect temperature sensor type connected: Cylinder temperature sensor type used as collector temperature sensor (T1).

What causes the Worcester Bosch Fault 51 / 123 fault?

This fault occurs when the boiler's internal control board detects an electrical signal from a temperature sensor that doesn't match the expected specifications. The most common reason is that an incompatible sensor has been installed during a recent repair, or a wiring fault is causing the boiler to misidentify the part. Because the boiler cannot accurately monitor heat levels with the wrong sensor, it stops operating as a safety precaution.

Typical repair cost £90-180 (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 Fault 51 / 123 fault code

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

Source

Worcester Bosch Fault 51 / 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 27 April 2026