21 Medium

Worcester Bosch 21 Fault Code

The Worcester Bosch 21 fault code means: your boiler's wall-mounted heating controller is having trouble communicating with the temperature sensor, which may prevent your heating from turning on correctly.. 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: Fault 03 Room temp sensor faulty: There is a short circuit on the room temperature sensor built into the FW 100
Typical repair cost £150-250 (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 21 fault code

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

Source

Worcester Bosch 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 7 July 2026