101 Boiler Fault Code: What It Means on Every Brand

The 101 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: Stop & check now 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 101 means	Severity	DIY?
AAriston	Fault code 101 on an Ariston boiler indicates that the central heating circuit has reached a dangerously high temperature and the boiler has cut out as a safety measure. Full guide →	Stop & check now	Some checks
AATAG	Fault code 101 on an ATAG boiler signals that the unit has detected — or suspects it has detected — an over-temperature condition. Full guide →	Lockout — engineer likely	No
CChaffoteaux	Fault code 101 on a Chaffoteaux boiler signals that the water temperature inside the primary circuit has climbed to an unsafe level, triggering the boiler's built-in safety lockout. Full guide →	Lockout — engineer likely	No

101 brand by brand

Ariston 101

Fault code 101 on an Ariston boiler indicates that the central heating circuit has reached a dangerously high temperature and the boiler has cut out as a safety measure. The overheating thermostat or NTC sensor positioned at the primary heat exchanger outlet — which triggers at around 102°C — has detected excessive coolant temperature and signalled the boiler to shut down before damage can occur. Until the underlying cause is identified and resolved, the boiler will not restart normally.

Causes, fixes & costs for Ariston 101 → · All Ariston codes

ATAG 101

Fault code 101 on an ATAG boiler signals that the unit has detected — or suspects it has detected — an over-temperature condition. Specifically, the boiler's control system monitors the flow sensor (T1) and return sensor (T2), and if either reads above 100 °C for around three seconds, the boiler locks out and displays this code. Because uncontrolled overheating carries serious safety risks, the boiler shuts itself down as a precaution until the underlying cause is identified and resolved. In some cases a faulty sensor gives a false high reading, triggering the lockout even when actual water temperatures are normal. A secondary — less authoritative — interpretation from some trade sources associates code 101 with an ignition sequence problem, but ATAG's own documentation is clear that the primary definition relates to the T1/T2 over-temperature condition.

Causes, fixes & costs for ATAG 101 → · All ATAG codes

Chaffoteaux 101

Fault code 101 on a Chaffoteaux boiler signals that the water temperature inside the primary circuit has climbed to an unsafe level, triggering the boiler's built-in safety lockout. In plain terms, the heat exchanger has got too hot — either because heat is not being carried away fast enough by circulating water, or because a sensor or component has reported a dangerously high reading. The boiler shuts itself down automatically to prevent damage. You may be able to reset it once or twice, but if the code returns, an underlying fault needs professional attention.

Causes, fixes & costs for Chaffoteaux 101 → · All Chaffoteaux codes

Frequently asked questions

Is the 101 fault code the same on every boiler?

No. 101 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 101 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. The underlying repair usually needs a registered engineer. Never open the boiler casing yourself.

Who should I call about a 101 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.