118 Boiler Fault Code: What It Means on Every Brand

The 118 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 118 means	Severity	DIY?
AAriston	Fault code 118 on Ariston boilers most commonly signals that the boiler's heating delivery (flow) temperature probe has returned a reading outside its expected range, prompting the boiler… Full guide →	Lockout — engineer likely	No
AATAG	Fault code 118 on an Atag boiler signals that the central heating system pressure has dropped too low — typically below 0.8–1 bar. Full guide →	Engineer may be needed	Some checks
CChaffoteaux	Fault code 118 on Chaffoteaux boilers signals that the primary circuit water pressure has dropped too low — typically below 0.5–1.0 bar depending on the model. Full guide →	Engineer may be needed	Some checks

118 brand by brand

Ariston 118

Fault code 118 on Ariston boilers most commonly signals that the boiler's heating delivery (flow) temperature probe has returned a reading outside its expected range, prompting the boiler to shut down as a precaution. This probe — sometimes called an NTC thermistor or flow sensor — measures the temperature of hot water leaving the heat exchanger on its way to the heating circuit. When the boiler cannot get a reliable reading from it, it cannot safely regulate heat output and will lock out. A secondary, less precise interpretation occasionally cited on general advice sites links code 118 to low system water pressure, though this more accurately describes Ariston code 108. If your boiler display shows 118, focus first on the temperature probe, but also glance at your pressure gauge — if it reads below 1 bar, low pressure could be a contributing factor worth ruling out before calling an engineer.

Causes, fixes & costs for Ariston 118 → · All Ariston codes

ATAG 118

Fault code 118 on an Atag boiler signals that the central heating system pressure has dropped too low — typically below 0.8–1 bar. The boiler detects insufficient water pressure in the sealed system and shuts itself down as a precaution, cutting off both heating and hot water until the issue is resolved. In some cases, particularly on models such as the iC Econ 27, the code can also point to a problem with the water pressure sensor itself rather than genuine low pressure in the system.

Causes, fixes & costs for ATAG 118 → · All ATAG codes

Chaffoteaux 118

Fault code 118 on Chaffoteaux boilers signals that the primary circuit water pressure has dropped too low — typically below 0.5–1.0 bar depending on the model. When the boiler detects this, it shuts down as a protective measure to prevent damage to the heat exchanger and pump. You will usually lose both heating and hot water, and the 118 code will appear on the boiler's digital display. The fix is often as simple as topping up the system pressure via the filling loop, but if pressure keeps falling the underlying cause — a leak, a failed expansion vessel, or a faulty pressure relief valve — must be investigated by a Gas Safe registered engineer. Note: fault code numbering varies across Chaffoteaux's range; the numeric 118 code is associated with modern Ariston-platform Chaffoteaux boilers such as the iC and iC Economiser Plus. Older models (e.g. Britony Combi SE) used LED flash sequences rather than a numeric display — always cross-reference your model's manual if unsure.

Causes, fixes & costs for Chaffoteaux 118 → · All Chaffoteaux codes

Frequently asked questions

Is the 118 fault code the same on every boiler?

No. 118 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 118 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 118 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.