10 Boiler Fault Code: What It Means on Every Brand

The 10 fault code appears on 3 brands — ATAG, Intergas and Viessmann — 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 10 means	Severity	DIY?
AATAG	Fault code 10 on ATAG boilers signals a problem with the outdoor temperature sensor. Full guide →	Engineer may be needed	Some checks
IIntergas	Fault code 10 on an Intergas boiler indicates that the PCB has detected a problem with the S1 sensor — the temperature sensor responsible for monitoring the flow of water leaving the heat… Full guide →	Lockout — engineer likely	No
VViessmann	Fault code 10 on Viessmann boilers signals that the boiler's control system has detected a short circuit in the outdoor temperature sensor circuit. Full guide →	Engineer may be needed	No

10 brand by brand

ATAG 10

Fault code 10 on ATAG boilers signals a problem with the outdoor temperature sensor. The boiler expects to read a valid signal from this sensor — used for weather compensation to fine-tune heating output based on external conditions — but is either receiving no signal, an out-of-range value, or detecting a short circuit. This is a blocking-type fault rather than a full lockout, meaning the boiler may continue to operate in a limited capacity and can clear itself if the underlying problem resolves. However, repeated occurrences will eventually lock the boiler out entirely. Note: on later-generation ATAG i Series models the same fault is logged as code 114 — the cause and fix are identical regardless of which number appears on your display.

Causes, fixes & costs for ATAG 10 → · All ATAG codes

Intergas 10

Fault code 10 on an Intergas boiler indicates that the PCB has detected a problem with the S1 sensor — the temperature sensor responsible for monitoring the flow of water leaving the heat exchanger. The PCB expects a continuous, plausible reading from this sensor; if the signal drops out entirely or falls outside acceptable limits, the boiler locks out and displays code 10. Worth noting: fault codes 10, 11, 12, 13, and 14 are all related to the heat exchanger sensor circuit. Codes 11–14 relate to the S2 return sensor and various out-of-range temperature conditions, but code 10 specifically points to an open or invalid signal from the S1 flow sensor. None of these codes can be permanently cleared by resetting alone — the underlying fault must be resolved first.

Causes, fixes & costs for Intergas 10 → · All Intergas codes

Viessmann 10

Fault code 10 on Viessmann boilers signals that the boiler's control system has detected a short circuit in the outdoor temperature sensor circuit. Because the boiler can no longer read a reliable outdoor temperature, it switches into a fixed 'constant operation' mode — essentially running blind without weather compensation data. A closely related code, 18, triggers the same constant-operation fallback but for the opposite electrical reason: an open circuit or broken cable in the outdoor sensor rather than a short circuit. If you see code 18, the diagnosis and fix process is almost identical to code 10, so the information on this page applies to both. The practical effect for the homeowner is often underwhelming heating performance: the boiler may underestimate how cold it is outside and reduce its output accordingly, leaving radiators lukewarm even though the boiler appears to be running normally.

Causes, fixes & costs for Viessmann 10 → · All Viessmann codes

Frequently asked questions

Is the 10 fault code the same on every boiler?

No. 10 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 10 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 10 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.