BoilerFaultCodesWorcester Bosch 5.b Fault Code
Also written as 5B — same fault.
The Worcester Bosch 5.b fault code means: your boiler's fan is running longer than it should after the heating has turned off, which is usually a sign of a communication error between internal components.. It appears on 1 Worcester Bosch model in our database.
Technical description: Fan over-run time
What causes the Worcester Bosch 5.b fault?
This fault occurs when the boiler's internal computer stays in a cooling phase for too long because it can no longer accurately track the fan's speed. The most common reason is a communication breakdown where the control board receives incorrect signals from the fan, causing the system to keep the fan running as a safety precaution. Over time, heat stress or moisture can degrade the electrical connections or the sensitive circuitry within the fan motor itself.
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 5.b fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch 5.b 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