704 Boiler Fault Code: What It Means on Every Brand

The 704 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 704 means	Severity	DIY?
AAriston	Fault code 704 tells you that the boiler has detected a problem with the NTC thermistor (temperature sensor) fitted to the return pipe of Heating Zone 3. Full guide →	Engineer may be needed	No
CChaffoteaux	Fault code 704 on Chaffoteaux boilers indicates a problem with the return temperature sensor (NTC thermistor) on heating zone 3. Full guide →	Lockout — engineer likely	No

704 brand by brand

Ariston 704

Fault code 704 tells you that the boiler has detected a problem with the NTC thermistor (temperature sensor) fitted to the return pipe of Heating Zone 3. This sensor's job is to measure the temperature of water coming back into the boiler from Zone 3; when it gives an implausible reading or stops responding altogether, the boiler logs code 704 and may shut down that zone or lock out entirely. The fault is essentially a sensor or wiring issue rather than a gas or combustion problem. Note: if you own an older Ariston Clas Net One, Clas One, E-Combi (non-Evo), or E-System (non-Evo) model and see code 713 on your display, the diagnosis and repair process is identical — 713 is simply the equivalent code for those model families. Related zone sensor codes follow the same pattern: 711 covers the Zone 1 return sensor and 712 covers the Zone 2 return sensor, so all three are diagnosed and fixed in the same way.

Causes, fixes & costs for Ariston 704 → · All Ariston codes

Chaffoteaux 704

Fault code 704 on Chaffoteaux boilers indicates a problem with the return temperature sensor (NTC thermistor) on heating zone 3. This sensor monitors the temperature of water coming back to the boiler after it has circulated through the zone 3 heating circuit, helping the boiler maintain safe and efficient operation. When the boiler's control board receives a reading from this sensor that falls outside the expected range — or receives no signal at all — it logs code 704 and may lock out. Because zone 3 is specific to multi-zone installations, this fault is most commonly seen on Chaffoteaux models fitted with external zone modules (such as the Alixia, Inoa, Talia, and Pigma ranges) rather than straightforward single-zone combis. Codes 701–703 cover flow probe faults on zones 1–3 respectively, so if you are also seeing those codes it can help narrow down whether the issue lies with wiring, a zone module, or the main PCB.

Causes, fixes & costs for Chaffoteaux 704 → · All Chaffoteaux codes

Frequently asked questions

Is the 704 fault code the same on every boiler?

No. 704 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 704 fault myself?

Not usually. On every brand listed, 704 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 704 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.