BoilerFaultCodesWorcester Bosch 66C Fault Code
The Worcester Bosch 66C fault code means: your boiler has stopped working because it cannot confirm that the fan is safely pushing exhaust fumes out of the flue.. It appears on 1 Worcester Bosch model in our database.
Technical description: Differential fan pressure switch
What causes the Worcester Bosch 66C fault?
This fault occurs when the boiler’s safety sensors cannot detect any airflow, meaning it is unsure if dangerous exhaust fumes are being cleared. The most common reasons are a buildup of dirt in the fan, a blockage in the thin plastic tubes inside the casing, or the air pressure switch failing to click into place. Because the boiler cannot guarantee a safe exit for burnt gases, it shuts down immediately as a protective measure.
This issue is more frequent in autumn or after high winds, as debris such as fallen leaves or nesting material can become lodged in the external flue pipe.
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 66C fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch 66C 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