BoilerFaultCodesWorcester Bosch 51123 Fault Code
The Worcester Bosch 51123 fault code means: the system has detected that the wrong type of temperature sensor has been installed for your solar heating circuit.. It appears on 1 Worcester Bosch model in our database.
Technical description: Cylinder temperature sensor type used as collector temperature sensor (T1)
What causes the Worcester Bosch 51123 fault?
This fault happens because the control system has detected an electrical signal that doesn't match the expected part for your solar panels. It usually occurs if the wrong type of probe was fitted during a repair or if the internal resistor in the sensor has failed, causing it to send the wrong data to the boiler. As a result, the solar thermal system cannot accurately monitor the temperature at the roof panels.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
Find a Gas Safe engineer
Search the official register for a qualified engineer in your area.
Gas Safe Register →Worcester Bosch models that show the 51123 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch 51123 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