Fault 50 / 121 Low

Worcester Bosch Fault 50 / 121 Fault Code

Also written as FAULT50121 — same fault.

The Worcester Bosch Fault 50 / 121 fault code means: your solar heating system is unable to move water because the pump is stuck or there is an air bubble trapped inside the pipes.. It appears on 2 Worcester Bosch models in our database.

Monitor it

This is a minor fault. Keep an eye on it and mention it at your next annual boiler service. Reset the boiler and see if it clears.

Technical description: Solar pump jammed or air in system: Solar pump (SP) sticking due to physical blockage.

What causes the Worcester Bosch Fault 50 / 121 fault?

This fault occurs when the solar heating circuit is unable to circulate fluid correctly, usually because the pump has seized after a period of inactivity or an air pocket has become trapped in the pipework. Without this flow, the system cannot transfer heat from your roof panels to the water cylinder, causing the control unit to flag an error. It is often caused by debris build-up in the pump or a leak that has allowed air to enter the sealed system.

This fault often appears at the start of spring or during the first hot days of the year when the solar system attempts to start up after sitting idle throughout the winter.

Typical repair cost £120-280 (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 50 / 121 fault code

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

Source

Worcester Bosch Fault 50 / 121 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