105 Boiler Fault Code: What It Means on Every Brand

The 105 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 105 means	Severity	DIY?
AAriston	Fault code 105 on an Ariston boiler signals that the boiler has detected a dangerously large temperature gap between the flow pipe (water leaving the boiler) and the return pipe (water… Full guide →	Lockout — engineer likely	No
AATAG	Fault code 105 on an Atag boiler points to a problem with the condensate drainage system — typically a blocked or frozen condensate pipe that is preventing the boiler from expelling acidic… Full guide →	Engineer may be needed	Some checks
CChaffoteaux	Fault code 105 on Chaffoteaux boilers indicates one of two closely related problems depending on your specific model and age. Full guide →	Lockout — engineer likely	Some checks

105 brand by brand

Ariston 105

Fault code 105 on an Ariston boiler signals that the boiler has detected a dangerously large temperature gap between the flow pipe (water leaving the boiler) and the return pipe (water coming back). Specifically, this difference has exceeded 55°C, which tells the boiler that very little — or no — water is moving around the heating system. If the boiler registers this condition three times consecutively, it will lock out entirely as a safety measure. In simple terms: the boiler is producing heat, but that heat is not being carried away properly because water circulation is severely restricted or absent.

Causes, fixes & costs for Ariston 105 → · All Ariston codes

ATAG 105

Fault code 105 on an Atag boiler points to a problem with the condensate drainage system — typically a blocked or frozen condensate pipe that is preventing the boiler from expelling acidic waste water safely. On some Atag models the related code C105 can also indicate a failure to ignite or establish a flame, which may follow on from a condensate blockage once the pipe is cleared and air has entered the gas line. In either case the boiler shuts itself down as a protective measure until the underlying issue is resolved.

Causes, fixes & costs for ATAG 105 → · All ATAG codes

Chaffoteaux 105

Fault code 105 on Chaffoteaux boilers indicates one of two closely related problems depending on your specific model and age. On most units — particularly those running the Expert Control system — it signals a flow check failure, meaning the boiler has detected that water is not circulating correctly around the heating circuit. On some other Chaffoteaux models, the same code points to a fault in the supply (flow) temperature sensor circuit, caused by a short circuit or open break in the NTC sensor wiring. In both cases the boiler will lock out as a safety measure. Note that codes 103, 104, 105, 106, and 107 are all part of the same flow-check family on the Expert Control range, so if you see any of these, the diagnosis process is broadly the same. Always cross-reference with your own model's manual to confirm which interpretation applies to your appliance.

Causes, fixes & costs for Chaffoteaux 105 → · All Chaffoteaux codes

Frequently asked questions

Is the 105 fault code the same on every boiler?

No. 105 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 105 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 105 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.