A04 Boiler Fault Code: What It Means on Every Brand

The A04 fault code appears on 2 brands — Ferroli and Vokera — and it does not mean the same thing on each. The same two or three characters can signal a pressure problem on one boiler and an ignition fault on another, so identify your brand first and use the table below to jump to the right page.

Up to: Lockout — engineer likely 2 brands use this code Meaning differs by brand

Which brand is your boiler?

The brand is printed on the front casing. Tap your brand for the full causes, fixes and repair costs.

Brand	What A04 means	Severity	DIY?
FFerroli	The A04 is a lockout fault on Ferroli boilers, meaning the boiler has shut itself down and will not restart until you manually reset it. Full guide →	Lockout — engineer likely	No
VVokera	The A04 code on a Vokera boiler indicates that the system water pressure has dropped below the minimum level required for safe operation. Full guide →	Engineer may be needed	Some checks

A04 brand by brand

Ferroli A04

The A04 is a lockout fault on Ferroli boilers, meaning the boiler has shut itself down and will not restart until you manually reset it. In plain terms, your boiler has detected a serious problem with the way it is handling flue gases — either dangerously high flue gas temperatures, a fault with the flue gas removal system, or both occurring repeatedly. On most Ferroli models (including the Modena, Bluehelix, and i-Series ranges), A04 specifically means that the underlying F07 fault — high fume temperature detected by the flue probe — has been triggered three or more times within a 24-hour period. On Domiproject D and C models, it points directly to overheating of the flue gas thermostat. On the Optimax HE Plus 38C it indicates activation of the fume extraction duct safety device. In all cases the boiler is telling you it cannot safely remove combustion gases, and it will stay locked out until the root cause is resolved.

Causes, fixes & costs for Ferroli A04 → · All Ferroli codes

Vokera A04

The A04 code on a Vokera boiler indicates that the system water pressure has dropped below the minimum level required for safe operation. When cold and at rest, a Vokera boiler typically needs to read between 1 and 1.5 bar on the pressure gauge. If pressure falls significantly below this, the boiler will lock out and display A04 rather than continue running. Topping up the pressure via the filling loop will often clear the fault, but if pressure keeps dropping, an underlying leak or component fault needs professional attention.

Causes, fixes & costs for Vokera A04 → · All Vokera codes

Frequently asked questions

Is the A04 fault code the same on every boiler?

No. A04 means different things on different brands — see the table above. Always identify your boiler's brand (it is printed on the front casing) before acting on any advice about the code.

Can I fix a A04 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. Where the code is a simple pressure or reset condition, that may be all it needs. Never open the boiler casing yourself.

Who should I call about a A04 fault?

For gas boilers, a Gas Safe registered engineer — it is a legal requirement for any work on the appliance. If you smell gas at any point, leave the property and call the National Gas Emergency line on 0800 111 999.

General guidance only — not a substitute for professional advice, and not certified by a registered engineer. Any work on a gas appliance must be carried out by a Gas Safe registered engineer. If you smell gas, call 0800 111 999.