E40 Boiler Fault Code: What It Means on Every Brand

The E40 fault code appears on 4 brands — Baxi, Keston, Main and Potterton — and it does not mean the same thing on each. The same two or three characters can signal a pressure problem on one boiler and an ignition fault on another, so identify your brand first and use the table below to jump to the right page.

Up to: Lockout — engineer likely 4 brands use this code Meaning differs by brand

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 E40 means	Severity	DIY?
BBaxi	The E40 code appears on Baxi boilers when the PCB detects a problem with the central heating return NTC (negative temperature coefficient) thermistor — the sensor that continuously… Full guide →	Lockout — engineer likely	No
KKeston	The E40 fault code on Keston boilers signals that the system water pressure has climbed above the acceptable upper limit. Full guide →	Lockout — engineer likely	Some checks
MMain	The E40 fault code on Main boilers (which share their fault code platform with Baxi boilers) indicates a problem with the central heating return NTC (negative temperature coefficient)… Full guide →	Lockout — engineer likely	No
PPotterton	The E40 code on a Potterton boiler points to a problem with the central heating (CH) return thermistor — the sensor responsible for measuring the temperature of water as it flows back into… Full guide →	Engineer may be needed	No

E40 brand by brand

Baxi E40

The E40 code appears on Baxi boilers when the PCB detects a problem with the central heating return NTC (negative temperature coefficient) thermistor — the sensor that continuously monitors the temperature of water coming back from your radiators. When this sensor sends readings outside the expected range, or stops responding altogether, the boiler shuts down as a precaution to prevent overheating or unsafe operation. The boiler will remain locked out until the fault is properly diagnosed and cleared.

Causes, fixes & costs for Baxi E40 → · All Baxi codes

Keston E40

The E40 fault code on Keston boilers signals that the system water pressure has climbed above the acceptable upper limit. In plain terms, there is too much water in the heating circuit, or something is preventing the system from managing pressure correctly as the water heats up and expands. When E40 triggers, the boiler locks out and will not restart on its own — you must press the reset button once the underlying cause has been addressed. Ideal operating pressure on a Keston boiler should sit at around 1.0–1.5 bar when cold, typically settling near 1.3 bar.

Causes, fixes & costs for Keston E40 → · All Keston codes

Main E40

The E40 fault code on Main boilers (which share their fault code platform with Baxi boilers) indicates a problem with the central heating return NTC (negative temperature coefficient) thermistor. This sensor monitors the temperature of water flowing back into the boiler from your radiators. When the boiler's control system detects that this sensor is giving readings outside expected parameters — or no reading at all — it shuts the boiler down as a safety precaution. You'll typically find the boiler has locked out completely and won't fire until the fault is cleared. Note: E40 can carry different meanings on other brands such as Vokèra or Keston, so always confirm against your specific model's documentation.

Causes, fixes & costs for Main E40 → · All Main codes

Potterton E40

The E40 code on a Potterton boiler points to a problem with the central heating (CH) return thermistor — the sensor responsible for measuring the temperature of water as it flows back into the boiler from the heating circuit. This sensor continuously feeds temperature data to the PCB so the boiler can regulate output and keep water temperatures within the correct range (typically 60–80°C for central heating). When the return thermistor sends readings that fall outside expected parameters, or stops communicating altogether, the boiler flags E40 and may shut down to protect itself from overheating or running inefficiently.

Causes, fixes & costs for Potterton E40 → · All Potterton codes

Frequently asked questions

Is the E40 fault code the same on every boiler?

No. E40 means different things on different brands — see the table above. Always identify your boiler's brand (it is printed on the front casing) before acting on any advice about the code.

Can I fix a E40 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 E40 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.