A21 Boiler Fault Code: What It Means on Every Brand

The A21 fault code appears on 2 brands — Andrews and Ferroli — 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 A21 means	Severity	DIY?
AAndrews	The A21 code appears when your Worcester Bosch boiler loses its radio frequency (RF) communication link with the paired wireless thermostat or its receiver. Full guide →	Engineer may be needed	Some checks
FFerroli	The A21 is a lockout fault on Ferroli boilers, meaning the appliance has shut itself down completely and will not restart until the underlying problem is addressed and the boiler is… Full guide →	Lockout — engineer likely	No

A21 brand by brand

Andrews A21

The A21 code appears when your Worcester Bosch boiler loses its radio frequency (RF) communication link with the paired wireless thermostat or its receiver. Because the boiler can no longer receive heating demands from the thermostat, it stops firing — even if the room is cold. This is almost always a controls or signal issue rather than a fault inside the boiler itself. The A21 display can appear both on the boiler and on the thermostat handset. Note: if you have an Andrews Water Heater unit, fault code numbering differs significantly from Worcester Bosch — please consult your Andrews manual or contact Andrews directly, as this page covers the Worcester Bosch A21 specifically.

Causes, fixes & costs for Andrews A21 → · All Andrews codes

Ferroli A21

The A21 is a lockout fault on Ferroli boilers, meaning the appliance has shut itself down completely and will not restart until the underlying problem is addressed and the boiler is manually reset. It indicates that the boiler's control system has detected unacceptably poor combustion quality — in other words, the burner is not burning gas cleanly or efficiently enough to meet the boiler's safety thresholds. On models such as the DIVAtech and DOMIcondens F series, this lockout is triggered automatically after the related blocking fault (F20) has occurred six times within a ten-minute window, confirming the issue is persistent rather than a one-off blip. The root causes range from flue or condensate blockages through to a faulty ionisation electrode, gas valve, fan, or PCB.

Causes, fixes & costs for Ferroli A21 → · All Ferroli codes

Frequently asked questions

Is the A21 fault code the same on every boiler?

No. A21 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 A21 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 A21 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.