201 Boiler Fault Code: What It Means on Every Brand

The 201 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 201 means	Severity	DIY?
AAriston	Fault code 201 on an Ariston boiler signals a problem with the domestic hot water (DHW) NTC temperature sensor circuit. Full guide →	Lockout — engineer likely	No
AATAG	Fault code 201 on an Atag boiler indicates that the T3 NTC thermistor — the temperature sensor responsible for monitoring domestic hot water (DHW) or boiler flow temperature depending on… Full guide →	Lockout — engineer likely	No
CChaffoteaux	Fault code 201 on Chaffoteaux boilers indicates that the control board has detected a problem with the domestic hot water (DHW) NTC thermistor — the sensor responsible for monitoring the… Full guide →	Engineer may be needed	No

201 brand by brand

Ariston 201

Fault code 201 on an Ariston boiler signals a problem with the domestic hot water (DHW) NTC temperature sensor circuit. The boiler's control board has detected either a short circuit or an open circuit (break) in the wiring loop connected to the hot water sensor, meaning it can no longer get a reliable temperature reading from the DHW side. Without that data, the boiler cannot safely regulate hot water output, so it shuts down and locks out. In Ariston's fault numbering system, the leading digit '2' identifies the domestic hot water circuit as the affected group — so any code beginning with 2 points to the hot water side rather than heating or ignition. Code 201 specifically flags the NTC sensor's electrical circuit as the problem. This is a non-volatile lockout, meaning the boiler will not restart on its own — some form of intervention is needed, whether that's a simple reset or a component repair.

Causes, fixes & costs for Ariston 201 → · All Ariston codes

ATAG 201

Fault code 201 on an Atag boiler indicates that the T3 NTC thermistor — the temperature sensor responsible for monitoring domestic hot water (DHW) or boiler flow temperature depending on the model — is reporting values outside the acceptable operating range. The PCB is receiving a signal that suggests an open circuit, short circuit, or a reading that simply does not match expected parameters. On iC and iCE combi boilers, this relates specifically to the T3 hot water sensor; on iS and iR system and regular boilers, the T3 boiler sensor is the one flagged. In either case the boiler will typically lock out until the fault is resolved.

Causes, fixes & costs for ATAG 201 → · All ATAG codes

Chaffoteaux 201

Fault code 201 on Chaffoteaux boilers indicates that the control board has detected a problem with the domestic hot water (DHW) NTC thermistor — the sensor responsible for monitoring the temperature of your hot water supply. The boiler cannot obtain a valid reading from this sensor, either because the circuit is open (broken) or has developed a short circuit, so the boiler shuts down as a precautionary measure. Code 201 sits at the start of the 201–209 range, which covers DHW circuit anomalies across Chaffoteaux Mira, Urbia, Serelia, and Niagara C models. While 201 specifically flags the sensor signal fault itself, codes in this range all relate to the domestic hot water circuit — if your display shows a code between 201 and 209, check your model's manual alongside this guide to confirm the exact sub-fault.

Causes, fixes & costs for Chaffoteaux 201 → · All Chaffoteaux codes

Frequently asked questions

Is the 201 fault code the same on every boiler?

No. 201 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 201 fault myself?

Not usually. On every brand listed, 201 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 201 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.