High
120FF on Glow-worm Glow-worm Micron 100FF 120FF

The code you are seeing refers to the technical heat output rating of your specific boiler model rather than a fault error, though if your boiler has stopped working, it indicates an underlying failure that requires professional diagnosis.

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.

What the manual says

Manufacturer manual

NOMINAL kW HEAT INPUT(NETT) Min. 33.43 Medium 36.52 Max. 39.6

From this model's manual
Verified against the Glow Worm Micron 100ff 120ff installation & service manual, page 7.
Plain-English guidance below. Severity, DIY steps, costs and parts are our interpretation to help you act, not text from the manual.

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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