E125 Boiler Fault Code: What It Means on Every Brand

E125 is a primary-circulation fault on Baxi, Potterton and Main: the control board fired the burner but saw the water temperature behave as though it was not moving. The suspects are the same on all three — a pump that has seized or slowed, a sludged filter or heat exchanger, an airlock after work on the system, or a stuck diverter valve. Sludge management is the recurring theme; a magnetic filter and inhibitor prevent most repeat E125s.

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 E125 means	Severity	DIY?
BBaxi	The E125 code tells you that the boiler's PCB has detected a problem with primary water circulation — essentially, it believes water is not moving through the heat exchanger at the rate it… Full guide →	Lockout — engineer likely	Some checks
MMain	The E125 code on a Main boiler signals a primary circulation fault — the boiler's PCB has detected that water is not moving through the internal circuit as it should. Full guide →	Lockout — engineer likely	Some checks
PPotterton	The E125 code appears on Potterton boilers when the appliance detects a problem with water circulation through the primary circuit. Full guide →	Lockout — engineer likely	No

E125 brand by brand

Baxi E125

The E125 code tells you that the boiler's PCB has detected a problem with primary water circulation — essentially, it believes water is not moving through the heat exchanger at the rate it should be. When flow stalls or water temperature climbs too fast, the board triggers a safety lockout to prevent the heat exchanger and other internal components from being damaged by overheating. The boiler will stop firing completely and show E125 on the display, often alongside a flashing red light. You won't get heating or hot water until the underlying cause is resolved.

Causes, fixes & costs for Baxi E125 → · All Baxi codes

Main E125

The E125 code on a Main boiler signals a primary circulation fault — the boiler's PCB has detected that water is not moving through the internal circuit as it should. Specifically, the board is watching for two things: the flow temperature failing to rise by at least 1°C within 15 seconds of the burner igniting, or the flow temperature overshooting your set temperature by 30°C or more on two occasions within a 10-minute window. Either pattern tells the PCB that heat is building up without being carried away, so it shuts the boiler down to prevent damage to internal components. Main boilers share their fault code system with Baxi and Potterton, as all three brands sit under the BDR Thermea Group umbrella.

Causes, fixes & costs for Main E125 → · All Main codes

Potterton E125

The E125 code appears on Potterton boilers when the appliance detects a problem with water circulation through the primary circuit. The boiler's controls monitor how quickly the water temperature changes during operation — if the temperature fails to rise by at least 1°C within 15 seconds, or spikes more than 30°C above the target temperature on two occasions, the boiler concludes that water is not moving through the system as it should. To protect itself from overheating, the boiler shuts down and displays E125. You may notice cold radiators, intermittent heating, unusual noises from the boiler, or repeated shutdowns before the lockout occurs.

Causes, fixes & costs for Potterton E125 → · All Potterton codes

Frequently asked questions

Is the E125 fault code the same on every boiler?

Broadly, yes — on Baxi, Main and Potterton the E125 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 E125 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 E125 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.