712 Boiler Fault Code: What It Means on Every Brand

The 712 fault code appears on 2 brands — Ariston 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 2 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 712 means	Severity	DIY?
AAriston	Fault code 712 on Ariston boilers indicates that the control board has detected a problem with the Zone 2 return temperature sensor — the NTC (Negative Temperature Coefficient) thermistor… Full guide →	Lockout — engineer likely	No
CChaffoteaux	Fault code 712 on a Chaffoteaux boiler signals that the control board has detected a problem with the return water temperature sensor on Zone 2 of a multi-zone heating system. Full guide →	Engineer may be needed	No

712 brand by brand

Ariston 712

Fault code 712 on Ariston boilers indicates that the control board has detected a problem with the Zone 2 return temperature sensor — the NTC (Negative Temperature Coefficient) thermistor responsible for monitoring the temperature of water coming back from the second heating zone. The boiler's electronics have either received a reading that falls outside the expected range, or the sensor circuit has gone open or short-circuit entirely. Until the control board can trust the readings from this sensor, the boiler locks out to protect the system. Note: on Ariston Clas Net One, Clas One, E-Combi, and E-System models, the equivalent code is 702, which points to the same type of return sensor fault on those platforms.

Causes, fixes & costs for Ariston 712 → · All Ariston codes

Chaffoteaux 712

Fault code 712 on a Chaffoteaux boiler signals that the control board has detected a problem with the return water temperature sensor on Zone 2 of a multi-zone heating system. The sensor — a thermistor — may be producing a reading that falls outside the expected range, or the boiler's electronics have detected either an open circuit (a broken or disconnected wire) or a short circuit (a direct electrical fault within the sensor or its wiring). Until the issue is resolved, the boiler cannot reliably monitor the return temperature on Zone 2, which prevents safe, accurate heating control. This code sits within a wider group of multi-zone sensor faults: codes 701–703 relate to flow sensor anomalies on Zones 1–3, while codes 711–713 cover return sensor anomalies on the same zones. Code 711 therefore points to Zone 1's return sensor, 712 to Zone 2, and 713 to Zone 3. If your boiler is a single-zone combi rather than a multi-zone system, code 712 may carry a different meaning — always cross-check with your specific model's documentation.

Causes, fixes & costs for Chaffoteaux 712 → · All Chaffoteaux codes

Frequently asked questions

Is the 712 fault code the same on every boiler?

No. 712 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 712 fault myself?

Not usually. On every brand listed, 712 points to a fault that needs a suitably registered engineer to diagnose and repair. A single reset to rule out a transient glitch is reasonable; repeated resets are not.

Who should I call about a 712 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.