E109 Boiler Fault Code: What It Means on Every Brand

The E109 fault code appears on 4 boiler brands — Baxi, Main, Navien and Potterton — 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 4 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 E109 means	Severity	DIY?
BBaxi	The E109 fault code appears on Baxi boilers when the appliance detects poor or insufficient water circulation within the heating system. Full guide →	Lockout — engineer likely	Some checks
MMain	The E109 code on Main boilers indicates a pre-circulation fault — the boiler has detected inadequate water flow through the heating system before or during the start of a heating cycle. Full guide →	Lockout — engineer likely	Some checks
NNavien	The E109 code on a Navien boiler indicates that the fan motor is not reaching the correct operating speed. Full guide →	Lockout — engineer likely	No
PPotterton	The E109 code appears on Potterton boilers when the appliance detects inadequate water movement through the heating system — either during the pre-circulation check that runs before the… Full guide →	Lockout — engineer likely	Some checks

E109 brand by brand

Baxi E109

The E109 fault code appears on Baxi boilers when the appliance detects poor or insufficient water circulation within the heating system. In practical terms, the boiler is struggling to move water around the pipework and radiators at the rate it expects. On certain Baxi models this is also described as a pre-circulation fault — the boiler runs a short circulation check before firing, and if water movement is inadequate during that phase, E109 is triggered and the boiler locks out as a precaution. Left unresolved, poor circulation can lead to cold spots on radiators, reduced efficiency, and potential overheating of internal components.

Causes, fixes & costs for Baxi E109 → · All Baxi codes

Main E109

The E109 code on Main boilers indicates a pre-circulation fault — the boiler has detected inadequate water flow through the heating system before or during the start of a heating cycle. In plain terms, the boiler is struggling to confirm that water is moving around the system properly, so it locks out as a precaution. Because Main and Baxi share the same BDR Thermea platform, the E109 code behaves identically across both brands. Left unresolved, poor circulation can cause cold spots on radiators, increased energy use, and potential overheating of boiler components.

Causes, fixes & costs for Main E109 → · All Main codes

Navien E109

The E109 code on a Navien boiler indicates that the fan motor is not reaching the correct operating speed. Your boiler's fan is responsible for drawing in fresh air for combustion and expelling harmful flue gases safely outside your home. If the control board detects that the fan isn't spinning at the required speed, it triggers a safety lockout and prevents the boiler from firing. This protects your household from the risks of incomplete combustion and carbon monoxide build-up.

Causes, fixes & costs for Navien E109 → · All Navien codes

Potterton E109

The E109 code appears on Potterton boilers when the appliance detects inadequate water movement through the heating system — either during the pre-circulation check that runs before the burner fires, or while the boiler is operating. Because insufficient circulation can lead to dangerous overheating of the heat exchanger, the boiler locks itself out as a safety measure until the underlying problem is identified and resolved. In plain terms: the boiler is trying to push water around your system and is not getting the response it expects.

Causes, fixes & costs for Potterton E109 → · All Potterton codes

Frequently asked questions

Is the E109 fault code the same on every boiler?

Broadly, yes — on Baxi, Main, Navien and Potterton the E109 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 E109 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 E109 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.