104 Boiler Fault Code: What It Means on Every Brand

The 104 fault code appears on 3 boiler brands — Ariston, ATAG 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 3 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 104 means	Severity	DIY?
AAriston	Ariston fault code 104 is a safety lockout triggered when the boiler detects that water is not circulating properly around the heating system. Full guide →	Lockout — engineer likely	Some checks
AATAG	Fault code 104 on an Atag boiler indicates that the system water pressure has dropped below 1 bar, or that the boiler's controls have failed to detect any pressure increase from the pump… Full guide →	Lockout — engineer likely	Some checks
CChaffoteaux	Fault code 104 on Chaffoteaux boilers signals that the boiler's control system has detected inadequate water circulation in the primary heating circuit. Full guide →	Lockout — engineer likely	Some checks

104 brand by brand

Ariston 104

Ariston fault code 104 is a safety lockout triggered when the boiler detects that water is not circulating properly around the heating system. More specifically, the boiler's sensors register that the temperature difference between the flow and return pipework is climbing faster than it should — the threshold is typically a rise of more than 20°C per second. When water is moving freely, heat is carried away from the heat exchanger at a steady rate; if circulation is poor or absent, heat builds up rapidly and the boiler shuts down to protect itself. On some older Ariston models (including certain Genus variants), code 104 may also be linked to insufficient system water pressure rather than a flow-rate issue, and the boiler will not reset automatically until the underlying problem is corrected. Either way, the root cause is the same: water is not travelling around the circuit as the boiler expects.

Causes, fixes & costs for Ariston 104 → · All Ariston codes

ATAG 104

Fault code 104 on an Atag boiler indicates that the system water pressure has dropped below 1 bar, or that the boiler's controls have failed to detect any pressure increase from the pump after it has been running at full load for around 5 seconds. In simple terms, either there is not enough water pressure in the heating circuit, or the pump is not doing its job of circulating water effectively. The boiler locks out as a safety measure to prevent damage from running with insufficient pressure or flow.

Causes, fixes & costs for ATAG 104 → · All ATAG codes

Chaffoteaux 104

Fault code 104 on Chaffoteaux boilers signals that the boiler's control system has detected inadequate water circulation in the primary heating circuit. The boiler monitors how quickly the flow temperature rises: if the thermistor detects a temperature increase faster than roughly 20°C per second, it interprets this as a sign that water is not moving properly around the system — because without sufficient flow, heat builds up rapidly at the heat exchanger rather than being carried away. On some Chaffoteaux models, code 104 can also indicate a fault with the flow (feed) temperature sensor itself, either a short circuit or a broken sensor circuit. In both cases, the boiler locks out to prevent overheating or component damage. Always cross-reference with your specific model's manual, as Chaffoteaux have produced many different ranges over the years.

Causes, fixes & costs for Chaffoteaux 104 → · All Chaffoteaux codes

Frequently asked questions

Is the 104 fault code the same on every boiler?

Broadly, yes — on Ariston, ATAG and Chaffoteaux the 104 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 104 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. The underlying repair usually needs a registered engineer. Never open the boiler casing yourself.

Who should I call about a 104 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.