604 Boiler Fault Code: What It Means on Every Brand

The 604 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 604 means	Severity	DIY?
AAriston	Fault code 604 on Ariston boilers indicates that the fan is not reaching the speed required for safe operation, or that the Hall sensor responsible for monitoring fan speed has reported a… Full guide →	Lockout — engineer likely	No
CChaffoteaux	Fault code 604 on a Chaffoteaux boiler means the boiler's control board has detected that the fan is spinning too slowly — or not spinning at all — and has locked the boiler out as a… Full guide →	Lockout — engineer likely	No

604 brand by brand

Ariston 604

Fault code 604 on Ariston boilers indicates that the fan is not reaching the speed required for safe operation, or that the Hall sensor responsible for monitoring fan speed has reported a problem. The boiler's control system continuously checks that the fan is spinning at the correct rate before and during a heating or hot water cycle — if it falls short, the boiler locks out rather than risk combustion gases failing to vent properly. This is a safety shutdown, not a nuisance trip, so it should be taken seriously and investigated promptly.

Causes, fixes & costs for Ariston 604 → · All Ariston codes

Chaffoteaux 604

Fault code 604 on a Chaffoteaux boiler means the boiler's control board has detected that the fan is spinning too slowly — or not spinning at all — and has locked the boiler out as a safety precaution. The fan serves two vital purposes: it draws fresh air into the combustion chamber and expels the burned gases through the flue. Without adequate fan speed, safe combustion cannot take place, so the boiler refuses to fire. This code appears on Chaffoteaux models with a digital display (such as the Pigma, Inoa, and Talia ranges). If you have an older Chaffoteaux with LED flash codes rather than a numeric display, consult your model's specific documentation as the fault-indication system differs.

Causes, fixes & costs for Chaffoteaux 604 → · All Chaffoteaux codes

Frequently asked questions

Is the 604 fault code the same on every boiler?

Broadly, yes — on Ariston and Chaffoteaux the 604 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 604 fault myself?

Not usually. On every brand listed, 604 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 604 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.