205 Boiler Fault Code: What It Means on Every Brand

The 205 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 205 means	Severity	DIY?
AAriston	Fault code 205 on Ariston boilers signals that the domestic hot water (DHW) inlet temperature probe has lost its electrical connection to the boiler's control board — in other words, the… Full guide →	Lockout — engineer likely	No
CChaffoteaux	Fault code 205 on Chaffoteaux boilers signals that the control board has detected a problem with the temperature sensor monitoring the domestic hot water (DHW) inlet from a connected solar… Full guide →	Engineer may be needed	No

205 brand by brand

Ariston 205

Fault code 205 on Ariston boilers signals that the domestic hot water (DHW) inlet temperature probe has lost its electrical connection to the boiler's control board — in other words, the circuit between the sensor and the PCB is broken (open circuit). On models linked to a solar thermal system, this is specifically the temperature sensor positioned at the solar-side inlet of the DHW circuit. Without a reliable signal from this probe, the boiler cannot accurately monitor incoming hot water temperatures and locks out to protect the system. Ariston groups all DHW (sanitary hot water) circuit faults under the 2xx series: code 205 means the probe has gone open circuit, while the closely related code 206 means the same probe has short-circuited. Both point to the same sensor and wiring loom, so the diagnostic approach is identical.

Causes, fixes & costs for Ariston 205 → · All Ariston codes

Chaffoteaux 205

Fault code 205 on Chaffoteaux boilers signals that the control board has detected a problem with the temperature sensor monitoring the domestic hot water (DHW) inlet from a connected solar thermal system. The boiler expects a continuous, valid resistance reading from this NTC sensor; when that signal disappears entirely (open circuit — a broken wire or failed sensor) or collapses to near-zero (short circuit — wires touching or a sensor that has internally shorted), the board logs code 205 and may lock out to protect the system. The closely related code 206 refers specifically to the short-circuit version of the same sensor fault, so if you see 206 on your display the diagnosis and repair path is essentially identical. Because this fault sits within the solar circuit rather than the main gas-heating circuit, it will only appear on Chaffoteaux models plumbed into a solar thermal panel — if your boiler has no solar connection, double-check your model's manual, as code numbering can vary slightly across the Chaffoteaux range (many of which share fault-code logic with Ariston, given their shared parent group).

Causes, fixes & costs for Chaffoteaux 205 → · All Chaffoteaux codes

Frequently asked questions

Is the 205 fault code the same on every boiler?

No. 205 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 205 fault myself?

Not usually. On every brand listed, 205 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 205 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.