High Lockout
C1 on Worcester Bosch Greenstar 25 HE Combi

Your boiler has stopped working because the internal fan is not spinning fast enough to safely clear away exhaust fumes.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Call an engineer soon

Book a Gas Safe engineer within 24 to 48 hours. Your boiler may be unsafe or could break down completely if left.

Technical description

Fan speed too low.

What causes this fault?

This fault occurs when the boiler's control system detects that the fan is not spinning at the correct speed to safely expel waste gases through the flue. The most common reasons include a buildup of dust and debris within the fan motor, a loose electrical connection, or a failing control board that is unable to send the correct power signal to the fan.

This fault is slightly more common in winter when the boiler works harder and the fan runs for longer periods, which can cause aging motor bearings to seize or fail.

Engineer required

This is a high severity fault. Stop using the boiler and call a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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Related fault codes

C4View →C6View →A1View →

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