705 Boiler Fault Code: What It Means on Every Brand

The 705 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 705 means	Severity	DIY?
AAriston	On Ariston Sensys and multi-zone system boilers, the display can show what appears to be '705' but is actually the two-part code '7 05', indicating a fault with the flow temperature sensor… Full guide →	Lockout — engineer likely	No
CChaffoteaux	Code 705 appears on Chaffoteaux boilers fitted with multi-zone heating systems and signals that the control board has detected an invalid reading from one of the zone circuit NTC… Full guide →	Lockout — engineer likely	No

705 brand by brand

Ariston 705

On Ariston Sensys and multi-zone system boilers, the display can show what appears to be '705' but is actually the two-part code '7 05', indicating a fault with the flow temperature sensor on zone 5 of the heating system. The boiler has detected that the zone 5 NTC thermistor is either reading outside its expected range or has lost communication with the PCB entirely. As a precaution, the boiler will lock out to prevent any damage to that heating circuit. If your display shows '7 05' split across two segments or lines, this page applies directly to your situation. Note that standard three-digit Ariston fault codes typically fall in other documented series (such as the 101–116 or 301–307 ranges); a true standalone '705' is not listed in Ariston's published UK fault code library, so the two-part '7 05' interpretation is the most likely match.

Causes, fixes & costs for Ariston 705 → · All Ariston codes

Chaffoteaux 705

Code 705 appears on Chaffoteaux boilers fitted with multi-zone heating systems and signals that the control board has detected an invalid reading from one of the zone circuit NTC temperature sensors — either an open circuit (broken connection) or a short circuit within the sensor itself. The 700-series fault block as a whole covers zone flow and return sensor anomalies across zones 1 to 3: codes 701–703 relate to zone flow (outflow) probes, code 704 covers the zone 3 return sensor, and codes 705–707 extend to further zone sensor variants in the same group. When any of these sensors reports an out-of-range or absent signal, the boiler triggers a safety shutdown to prevent the heating circuit from running blind without reliable temperature data. This code is found on newer Chaffoteaux models with a numeric display — such as the Talia, Pigma, Inoa, and Niagara ranges — rather than older LED-flash models like the Britony Combi SE.

Causes, fixes & costs for Chaffoteaux 705 → · All Chaffoteaux codes

Frequently asked questions

Is the 705 fault code the same on every boiler?

Broadly, yes — on Ariston and Chaffoteaux the 705 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 705 fault myself?

Not usually. On every brand listed, 705 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 705 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.