203 Boiler Fault Code: What It Means on Every Brand

The 203 fault code appears on 3 brands — Ariston, ATAG 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 3 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 203 means	Severity	DIY?
AAriston	Fault code 203 on Ariston boilers indicates that the control board (PCB) has lost its signal from the hot water cylinder's NTC temperature sensor — in plain terms, the sensor has gone open… Full guide →	Engineer may be needed	No
AATAG	Fault code 203 on an Atag boiler indicates that the boiler's control system has detected a problem with the tank probe — the sensor responsible for monitoring the temperature of the hot… Full guide →	Lockout — engineer likely	No
CChaffoteaux	Fault code 203 on a Chaffoteaux boiler indicates that the boiler has detected a problem with the hot water cylinder temperature sensor — known as the cylinder or 'ballon' probe. Full guide →	Engineer may be needed	No

203 brand by brand

Ariston 203

Fault code 203 on Ariston boilers indicates that the control board (PCB) has lost its signal from the hot water cylinder's NTC temperature sensor — in plain terms, the sensor has gone open circuit and the boiler can no longer read the cylinder temperature. This sits within Ariston's 200-series codes, which all relate to the domestic hot water (DHW) circuit and solar thermal system. The closely related code 204 points to the same sensor but in a different failure mode: a short circuit rather than a loss of contact. Code 203 is classified as a non-volatile fault, meaning it will not clear on its own — the boiler needs either a manual reset or a repair before it will resume normal operation. Because this fault depends on a separate hot water storage cylinder, it only applies to Ariston system boilers paired with a cylinder; it will not appear on combi boilers.

Causes, fixes & costs for Ariston 203 → · All Ariston codes

ATAG 203

Fault code 203 on an Atag boiler indicates that the boiler's control system has detected a problem with the tank probe — the sensor responsible for monitoring the temperature of the hot water storage tank or heat store in your system. When this sensor returns a reading that is outside the expected range, or stops responding altogether, the boiler logs code 203 and may lock out to prevent unsafe operation. This code is closely related to code 201, which covers the domestic hot water (DHW) probe directly on the boiler; code 203 specifically relates to the external tank or cylinder sensor used in system or heat-store configurations. If you see either code, the diagnostic approach is broadly the same.

Causes, fixes & costs for ATAG 203 → · All ATAG codes

Chaffoteaux 203

Fault code 203 on a Chaffoteaux boiler indicates that the boiler has detected a problem with the hot water cylinder temperature sensor — known as the cylinder or 'ballon' probe. This NTC thermistor sits on or within the hot water storage cylinder and continuously reports the stored water temperature back to the boiler's control board. When the boiler receives a reading that is out of range — whether implausibly low (suggesting an open circuit or disconnection), implausibly high (suggesting a short circuit), or simply absent — it logs fault 203 and suspends normal operation to prevent the cylinder being heated incorrectly. Because this sensor only exists on systems that include a separate hot water storage cylinder, fault 203 is exclusively a system boiler issue and will not appear on Chaffoteaux combi boilers.

Causes, fixes & costs for Chaffoteaux 203 → · All Chaffoteaux codes

Frequently asked questions

Is the 203 fault code the same on every boiler?

No. 203 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 203 fault myself?

Not usually. On every brand listed, 203 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 203 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.