303 Boiler Fault Code: What It Means on Every Brand

The 303 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 303 means	Severity	DIY?
AAriston	Fault code 303 on an Ariston boiler indicates an internal fault with the main Printed Circuit Board (PCB) — the electronic control unit that manages virtually every function of your boiler. Full guide →	Lockout — engineer likely	No
AATAG	Fault code 303 on an Atag boiler signals a problem with the PCB (Printed Circuit Board) — the central control unit that acts as the boiler's brain. Full guide →	Lockout — engineer likely	No
CChaffoteaux	Fault code 303 on a Chaffoteaux boiler indicates one of two closely related problems depending on the model generation and firmware version. Full guide →	Lockout — engineer likely	No

303 brand by brand

Ariston 303

Fault code 303 on an Ariston boiler indicates an internal fault with the main Printed Circuit Board (PCB) — the electronic control unit that manages virtually every function of your boiler. This is part of Ariston's 300-series codes, which as a group relate to the boiler's main PCB and electronic control systems. When this code appears, the boiler may become unresponsive to controls or lock out entirely. Note: a small number of online sources incorrectly describe 303 as a return temperature sensor fault — that is wrong. The return sensor fault on Ariston boilers is code 112. Code 303 is definitively a main PCB fault, as confirmed by Ariston's own published fault code tables.

Causes, fixes & costs for Ariston 303 → · All Ariston codes

ATAG 303

Fault code 303 on an Atag boiler signals a problem with the PCB (Printed Circuit Board) — the central control unit that acts as the boiler's brain. Every component in the boiler communicates through the PCB, so when it develops a fault, the boiler cannot operate safely and will lock out. The code can be triggered by a genuinely failed PCB, but also by wiring faults, moisture ingress, earthing problems, or issues with flame detection circuitry that feed signals into the board — so a Gas Safe engineer will investigate systematically before condemning the PCB itself.

Causes, fixes & costs for ATAG 303 → · All ATAG codes

Chaffoteaux 303

Fault code 303 on a Chaffoteaux boiler indicates one of two closely related problems depending on the model generation and firmware version. On many units it signals a detected anomaly on the main PCB (printed circuit board), causing the boiler to lock out and refuse to restart. On other models — particularly older ones — code 303 acts as an overheating or poor-circulation safety lockout, triggered when the primary circuit water temperature climbs above safe thresholds (around 95°C) or when flow through the heat exchanger becomes insufficient. Both scenarios end the same way: heating and hot water cut out and the boiler displays 303. Because the two root causes share several upstream triggers — a seized pump, trapped air, or a failing NTC sensor can stress the PCB and cause overheating simultaneously — the diagnostic steps overlap significantly. Treat this as a serious fault requiring prompt attention.

Causes, fixes & costs for Chaffoteaux 303 → · All Chaffoteaux codes

Frequently asked questions

Is the 303 fault code the same on every boiler?

Broadly, yes — on Ariston, ATAG and Chaffoteaux the 303 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 303 fault myself?

Not usually. On every brand listed, 303 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 303 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.