304 Boiler Fault Code: What It Means on Every Brand

The 304 fault code appears on 3 brands — Ariston, ATAG and Chaffoteaux — 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 3 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 304 means	Severity	DIY?
AAriston	Ariston fault code 304 is a safety lockout triggered when the boiler has been reset too many times in a short period. Full guide →	Lockout — engineer likely	No
AATAG	Fault code 304 on ATAG boilers (including the iC, iS, and iR ranges) means the boiler has been reset too many times within a 15-minute window and has locked itself out as a safety measure. Full guide →	Lockout — engineer likely	No
CChaffoteaux	Fault code 304 on a Chaffoteaux boiler is a deliberate safety lockout that activates when the reset button has been pressed more than five times within a 15-minute period. Full guide →	Engineer may be needed	Some checks

304 brand by brand

Ariston 304

Ariston fault code 304 is a safety lockout triggered when the boiler has been reset too many times in a short period. Specifically, Ariston's design allows up to five resets within any 15-minute window — if a sixth reset is attempted within that same window, the boiler locks out and displays code 304. This is the boiler's way of telling you that repeatedly pressing reset is not solving an underlying problem, and that continuing to do so would be unsafe. Note: on certain Ariston model ranges, code 304 has also been documented as a domestic hot water (DHW) temperature sensor fault — where the sensor responsible for measuring hot water temperature is reading outside its expected range. If your boiler has not been repeatedly reset and is showing 304, it is worth considering the DHW sensor as a possible cause. In the vast majority of cases, however, 304 indicates the too-many-resets lockout described above.

Causes, fixes & costs for Ariston 304 → · All Ariston codes

ATAG 304

Fault code 304 on ATAG boilers (including the iC, iS, and iR ranges) means the boiler has been reset too many times within a 15-minute window and has locked itself out as a safety measure. Crucially, 304 is a secondary lockout code rather than the root fault — it is the boiler's way of telling you that an underlying problem kept causing it to trip, and that repeatedly pressing reset is not solving it. Common companion faults that trigger repeated resets include ignition failures (such as fault 501, 'no flame detected'), low system pressure, and gas supply interruptions. Identifying and fixing that underlying issue is the only lasting solution.

Causes, fixes & costs for ATAG 304 → · All ATAG codes

Chaffoteaux 304

Fault code 304 on a Chaffoteaux boiler is a deliberate safety lockout that activates when the reset button has been pressed more than five times within a 15-minute period. The boiler recognises this pattern as a sign that something more serious may be wrong and refuses to accept any further reset attempts from the front panel. Importantly, code 304 is not itself a component failure — it is the boiler's way of telling you to stop repeatedly restarting it and find out why it keeps tripping in the first place. The underlying problem could be anything from a persistent ignition fault to a flame-sensing issue, and that root cause is what genuinely needs addressing.

Causes, fixes & costs for Chaffoteaux 304 → · All Chaffoteaux codes

Frequently asked questions

Is the 304 fault code the same on every boiler?

No. 304 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 304 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 304 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.