702 Boiler Fault Code: What It Means on Every Brand

The 702 fault code appears on 2 boiler brands — Ariston and Chaffoteaux — and on all of them it points to the same kind of problem. The details still differ by brand (thresholds, sub-codes, reset procedure), so use the table below to jump to the page for your boiler.

Lockout — engineer likely 2 brands use this code

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 702 means	Severity	DIY?
AAriston	Fault code 702 on Ariston boilers has two distinct meanings depending on your model. Full guide →	Lockout — engineer likely	No
CChaffoteaux	Fault code 702 on a Chaffoteaux boiler indicates that the control board is not receiving a valid signal from the NTC thermistor (temperature-sensing resistor) that monitors the temperature… Full guide →	Lockout — engineer likely	No

702 brand by brand

Ariston 702

Fault code 702 on Ariston boilers has two distinct meanings depending on your model. On multi-zone models such as the E-Combi Evo, E-System EVO, Clas HE evo Combi and Clas HE evo System, it points to a problem with the Zone 2 flow or return temperature sensor — the component that monitors water temperature in the second heating zone (for example, where underfloor heating runs alongside radiators). On standard combi models without zone control, the same code can indicate a fault with the domestic hot water (DHW) flow sensor or turbine, meaning the boiler fails to detect when a tap is opened and will not attempt to ignite the burner. In either case the boiler is protecting itself by locking out rather than operating with unreliable temperature or flow data. Note: on Clas Net One, Clas One, E-Combi and E-System ranges, the equivalent code is 712 — check your display carefully to confirm which code you are seeing.

Causes, fixes & costs for Ariston 702 → · All Ariston codes

Chaffoteaux 702

Fault code 702 on a Chaffoteaux boiler indicates that the control board is not receiving a valid signal from the NTC thermistor (temperature-sensing resistor) that monitors the temperature of water returning from the Zone 2 heating circuit. When this signal is absent or falls outside expected parameters, the boiler locks out as a precaution to prevent unsafe operation. This code is most commonly seen on multi-zone Chaffoteaux system boilers — such as the Talia System and similar zoned models — rather than on standard single-zone combi units. If your boiler is a single-zone combi, double-check the fault code against your model's manual, as code numbering can differ across the Chaffoteaux range.

Causes, fixes & costs for Chaffoteaux 702 → · All Chaffoteaux codes

Frequently asked questions

Is the 702 fault code the same on every boiler?

Broadly, yes — on Ariston and Chaffoteaux the 702 code points to the same class of fault, and the recommended response is similar. Model-specific details such as sub-codes and reset procedures still vary, so follow the page for your own brand.

Can I fix a 702 fault myself?

Not usually. On every brand listed, 702 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 702 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.