Emergency
F21 on Ferroli Optimax HE 31 C

Your boiler has detected an issue with the air intake or exhaust system, meaning it cannot safely burn the gas required to heat your home.

Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed

Act immediately

Turn off your boiler and stop using gas appliances. Call a Gas Safe engineer now. This fault can be dangerous if ignored.

What the manual says

Manufacturer manual

Combustion failure in the flue

From this model's manual
Verified against the Ferroli/Ferroli Optimax He 31 C 4726742 installation & service manual.
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 emergency 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

F05View →F37View →

All 32 fault codes on the Optimax HE 31 C

Seeing a different code? Every code this model can display, in one place.

A01A02A03A04A05A06A41F04F05F07F10F11F12F13F14F15F20F24F34F35F37F39F40F41F42F43F47F50F51Low system pressureNo burner ignition

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