Medium Warning
Flash 8 times per second on Worcester Bosch 24si II

Your boiler has been left in a service mode used for testing, meaning it is currently stuck running at a constant power level instead of operating normally.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

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

GV mode switch. Gas Valve mode switch on min or max.

What causes this fault?

This fault occurs when a manual switch inside the boiler has been left in a testing position, usually after a service or repair. It physically prevents the boiler from adjusting its flame size automatically, meaning the system cannot regulate the heat for your taps or radiators. While it is often just a switch left in the wrong position, it can also happen if the main control board becomes faulty and misreads the signal.

Engineer required

This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.

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