504 Boiler Fault Code: What It Means on Every Brand

The 504 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 504 means	Severity	DIY?
AAriston	Fault code 504 on an Ariston boiler indicates a flame lift problem — the burner flame has become unstable or has gone out unexpectedly during normal operation, and the boiler's control… Full guide →	Lockout — engineer likely	No
CChaffoteaux	Fault code 504 on a Chaffoteaux boiler signals a burner ionisation anomaly detected while the burner is running. Full guide →	Lockout — engineer likely	No

504 brand by brand

Ariston 504

Fault code 504 on an Ariston boiler indicates a flame lift problem — the burner flame has become unstable or has gone out unexpectedly during normal operation, and the boiler's control system has detected this loss of stable combustion. Rather than continuing to run unsafely, the boiler locks out and displays 504 to alert you. On 24 kW models this code is closely linked to the preceding 5P3 error: the boiler records consecutive 5P3 ignition instability events, and once ten occur in a row the display escalates to 504. The root cause on those units is often a deteriorated burner gasket that reduces the electrical conductivity between the burner and its fixing screws — effectively fooling the control system into thinking the flame is lifting. Because flame instability directly affects combustion safety, this fault should be investigated promptly rather than reset repeatedly.

Causes, fixes & costs for Ariston 504 → · All Ariston codes

Chaffoteaux 504

Fault code 504 on a Chaffoteaux boiler signals a burner ionisation anomaly detected while the burner is running. It typically appears after the boiler has already experienced six consecutive occurrences of fault code 5P3, meaning the control system has repeatedly failed to confirm a stable flame and has now locked the boiler out for safety. In plain terms, the boiler lit or attempted to light, but the ionisation electrode — the small sensor that detects the presence of a flame by measuring a tiny electrical current passing through it — could not confirm that a healthy flame was present. Without that confirmation, the boiler shuts off the gas supply automatically to keep your home safe. You will need to reset the boiler to attempt a restart, but if the fault returns promptly, further investigation by a Gas Safe registered engineer is needed.

Causes, fixes & costs for Chaffoteaux 504 → · All Chaffoteaux codes

Frequently asked questions

Is the 504 fault code the same on every boiler?

Broadly, yes — on Ariston and Chaffoteaux the 504 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 504 fault myself?

Not usually. On every brand listed, 504 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 504 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.