204 Boiler Fault Code: What It Means on Every Brand

The 204 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 204 means	Severity	DIY?
AAriston	Fault code 204 on Ariston boilers signals that the NTC thermistor (temperature sensor) monitoring the hot water storage cylinder has developed a short circuit. Full guide →	Engineer may be needed	No
CChaffoteaux	Fault code 204 on a Chaffoteaux boiler indicates that the PCB has detected an abnormal or absent signal from the solar collector NTC (negative temperature coefficient) temperature sensor. Full guide →	Lockout — engineer likely	No

204 brand by brand

Ariston 204

Fault code 204 on Ariston boilers signals that the NTC thermistor (temperature sensor) monitoring the hot water storage cylinder has developed a short circuit. Rather than sending a steady, reliable temperature reading, the sensor is producing a signal that the boiler's control board identifies as a short — so the boiler locks out to protect the system. On Ariston models fitted with solar thermal, code 204 can also point to a short-circuit fault in the solar collector temperature probe. Code 204 sits within the 200-series range, which covers domestic hot water circuit and solar system sensor faults. For context, the closely related code 203 indicates the same cylinder sensor has gone open-circuit (no contact at all), whereas 204 specifically means a short circuit is detected.

Causes, fixes & costs for Ariston 204 → · All Ariston codes

Chaffoteaux 204

Fault code 204 on a Chaffoteaux boiler indicates that the PCB has detected an abnormal or absent signal from the solar collector NTC (negative temperature coefficient) temperature sensor. This typically means the sensor has either developed an open circuit — where the electrical connection is broken — or a short circuit, where damaged wiring or moisture ingress causes an unintended connection. Because the boiler can no longer obtain reliable temperature data from the solar collector circuit, it locks out as a safety precaution. This is generally a non-volatile (permanent) fault, meaning a simple reset alone will not resolve it — the underlying cause must be identified and rectified before normal operation resumes.

Causes, fixes & costs for Chaffoteaux 204 → · All Chaffoteaux codes

Frequently asked questions

Is the 204 fault code the same on every boiler?

No. 204 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 204 fault myself?

Not usually. On every brand listed, 204 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 204 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.