BoilerFaultCodesWorcester Bosch Fault 12 Fault Code
Also written as FAULT12 — same fault.
The Worcester Bosch Fault 12 fault code means: your hot water tank has overheated and a safety switch has shut the system down to prevent damage.. It appears on 1 Worcester Bosch model in our database.
Technical description: The cylinder temperature is so high that the cylinder High Limit Thermal Cut-out has tripped.
What causes the Worcester Bosch Fault 12 fault?
This fault occurs when the water inside your hot water storage cylinder reaches an unsafely high temperature, triggering a safety switch to kill the power to the boiler. The most common reason is a mechanical failure in the motorised zone valve, which gets stuck in the 'open' position and continues to heat the tank even when the thermostat tells it to stop.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Worcester Bosch models that show the Fault 12 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch Fault 12 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