BoilerFaultCodesWorcester Bosch Fault 03 / 21 Fault Code
Also written as FAULT0321 — same fault.
The Worcester Bosch Fault 03 / 21 fault code means: your boiler's built-in thermostat controls has developed an electrical fault and can no longer tell how warm your room is.. It appears on 1 Worcester Bosch model in our database.
Technical description: Room temp sensor faulty: There is a short circuit on the room temperature sensor built into the FW 100.
What causes the Worcester Bosch Fault 03 / 21 fault?
This fault occurs when the electrical components inside the thermostat unit fail, preventing it from sending a clear signal to the boiler. The most common reason is an internal short circuit or a degraded sensor probe that can no longer accurately measure the air temperature in your home.
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 03 / 21 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch Fault 03 / 21 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