701 Boiler Fault Code: What It Means on Every Brand

The 701 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 701 means	Severity	DIY?
AAriston	Fault code 701 on Ariston boilers signals a problem with a flow or outgoing temperature sensor, though the exact sensor involved depends on your specific model. Full guide →	Lockout — engineer likely	No
CChaffoteaux	Fault code 701 appears on Chaffoteaux boilers that are set up in a multizone heating configuration. Full guide →	Engineer may be needed	No

701 brand by brand

Ariston 701

Fault code 701 on Ariston boilers signals a problem with a flow or outgoing temperature sensor, though the exact sensor involved depends on your specific model. On Clas Net One, Clas One, E-Combi, and E-System boilers it points to a defective Zone 1 flow (send) sensor — the thermistor that monitors the outgoing water temperature on your primary heating circuit. On E-Combi Evo, E-System Evo, Clas HE Evo Combi, and Clas HE Evo System models it instead flags a fault with the Zone 2 outgoing sensor. A small number of Ariston models use code 701 to indicate a DHW (domestic hot water) temperature sensor fault. In all cases, the boiler has detected that a sensor reading is missing, implausible, or outside the expected range, and it has locked out to protect the system. Confirm which definition applies to your boiler before proceeding — check the model name on the front panel and cross-reference with your user manual.

Causes, fixes & costs for Ariston 701 → · All Ariston codes

Chaffoteaux 701

Fault code 701 appears on Chaffoteaux boilers that are set up in a multizone heating configuration. It tells you the boiler's control board has detected something wrong with the NTC thermistor monitoring the flow (departure) temperature on Zone 1 — the pipe carrying hot water out to your first heating zone. NTC thermistors work by changing their electrical resistance as temperature rises or falls; the PCB reads those resistance values continuously to control heating output. When the signal falls outside the expected range — due to an open circuit, a short, or a wiring break — the boiler logs code 701 and may shut down that zone or lock out entirely. This fault belongs to a family of multizone sensor codes: 701 covers the Zone 1 flow sensor, 702 the Zone 2 flow sensor, and 703 the Zone 3 flow sensor. Codes 711, 712, and 713 cover the equivalent return sensors for each zone. Note that code 701 only applies to newer Chaffoteaux models with a digital display and multizone capability — such as the Urbia, Serelia, Mira C, and Expert Control ranges. Older models like the Britony Combi SE use a flashing LED system rather than numeric codes.

Causes, fixes & costs for Chaffoteaux 701 → · All Chaffoteaux codes

Frequently asked questions

Is the 701 fault code the same on every boiler?

No. 701 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 701 fault myself?

Not usually. On every brand listed, 701 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 701 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.