502 Boiler Fault Code: What It Means on Every Brand

The 502 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 502 means	Severity	DIY?
AAriston	Ariston fault code 502 indicates that the boiler's flame detection system is registering a flame signal at a point when the gas valve should be shut and no combustion should be occurring. Full guide →	Lockout — engineer likely	No
CChaffoteaux	Fault code 502 means the boiler's ionisation (flame detection) circuit is registering a flame signal at a point in the cycle when the gas valve should be fully closed. Full guide →	Lockout — engineer likely	No

502 brand by brand

Ariston 502

Ariston fault code 502 indicates that the boiler's flame detection system is registering a flame signal at a point when the gas valve should be shut and no combustion should be occurring. In plain terms, the boiler 'thinks' it can see a flame when there shouldn't be one. Because this contradicts a fundamental safety assumption — that closing the gas valve extinguishes the flame — the boiler locks out immediately to prevent any risk of uncontrolled combustion. This is a safety-critical fault that warrants prompt attention from a Gas Safe registered engineer rather than repeated reset attempts.

Causes, fixes & costs for Ariston 502 → · All Ariston codes

Chaffoteaux 502

Fault code 502 means the boiler's ionisation (flame detection) circuit is registering a flame signal at a point in the cycle when the gas valve should be fully closed. This is sometimes referred to as a 'ghost flame' or parasitic flame condition. Rather than a genuine flame burning where it shouldn't, this usually points to a faulty component sending a misleading signal — most commonly the ionisation electrode, the gas valve itself, or the PCB. Because the boiler cannot safely continue operating when its flame-detection logic is inconsistent, it shuts down and locks out until the underlying fault is resolved. Note that 502 is distinct from code 501, which indicates the opposite problem: a failure to detect a flame when one should be present. Both are lockout-type faults on Chaffoteaux digital-display models.

Causes, fixes & costs for Chaffoteaux 502 → · All Chaffoteaux codes

Frequently asked questions

Is the 502 fault code the same on every boiler?

Broadly, yes — on Ariston and Chaffoteaux the 502 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 502 fault myself?

Not usually. On every brand listed, 502 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 502 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.