BoilerFaultCodesWorcester Bosch 1-C1 Fault Code
Also written as 1C1 — same fault.
The Worcester Bosch 1-C1 fault code means: your boiler has detected that the flame has gone out or isn't burning correctly during operation, causing the system to shut down for safety.. It appears on 1 Worcester Bosch model in our database.
Technical description: Ionisation current
What causes the Worcester Bosch 1-C1 fault?
This fault occurs when the boiler's internal sensor can no longer detect a steady flame while the burner is active, leading the system to shut down for safety. The most common reasons are a worn-out sensing probe that can't 'see' the fire, a blockage in the gas supply, or an incorrect mixture of gas and air preventing a stable burn.
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 1-C1 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Worcester Bosch 1-C1 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