E04 Boiler Fault Code: What It Means on Every Brand

The E04 fault code appears on 4 brands — Biasi, Keston, Ravenheat and Warmflow — 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 E04 means	Severity	DIY?
BBiasi	The Biasi E04 fault (sometimes shown on the display as ER 04) signals that the boiler's primary circuit has encountered a problem with water circulation or system pressure. Full guide →	Lockout — engineer likely	Some checks
KKeston	The E04 code on a Keston boiler is a status notification rather than a fault with a specific component. Full guide →	Engineer may be needed	Some checks
RRavenheat	The E04 code on a Ravenheat boiler indicates that the PCB has detected a problem with the domestic hot water (DHW) NTC thermistor — the small temperature-sensing probe responsible for… Full guide →	Engineer may be needed	No
WWarmflow	The Warmflow E04 fault code carries two different meanings depending on which product range your boiler belongs to. Full guide →	Engineer may be needed	Some checks

E04 brand by brand

Biasi E04

The Biasi E04 fault (sometimes shown on the display as ER 04) signals that the boiler's primary circuit has encountered a problem with water circulation or system pressure. Specifically, the boiler's PCB has detected either insufficient water pressure, a failure in the pump's ability to circulate water around the system, or — less commonly — that the water pressure sensor is not connected or reading correctly. The result is a full boiler lockout: the boiler shuts itself down as a safety measure and will not fire until the underlying issue is resolved. This code is one of the more common faults on Biasi boilers and covers a range of causes from something as simple as a pressure drop to a worn pump or a cracked heat exchanger.

Causes, fixes & costs for Biasi E04 → · All Biasi codes

Keston E04

The E04 code on a Keston boiler is a status notification rather than a fault with a specific component. It appears when the mains power to the boiler has been switched off and back on while the boiler was already sitting in a lockout state from a pre-existing fault. In other words, E04 is telling you that someone cycled the power instead of pressing the dedicated reset button to clear the original lockout. The code itself does not tell you what caused the initial lockout — that underlying fault (such as low system pressure, an ignition failure, or an overheat condition) still needs to be identified and resolved before the boiler will run normally again.

Causes, fixes & costs for Keston E04 → · All Keston codes

Ravenheat E04

The E04 code on a Ravenheat boiler indicates that the PCB has detected a problem with the domestic hot water (DHW) NTC thermistor — the small temperature-sensing probe responsible for monitoring the temperature of your hot water. When this sensor sends a signal that falls outside the expected range, or stops responding altogether, the boiler logs E04 and shuts down to protect the system. This means your hot water is likely to stop working, though central heating may or may not be affected depending on the model. Note: Ravenheat Manufacturing Limited was dissolved in 2025, so parts availability is becoming increasingly limited — worth bearing in mind before committing to a repair.

Causes, fixes & costs for Ravenheat E04 → · All Ravenheat codes

Warmflow E04

The Warmflow E04 fault code carries two different meanings depending on which product range your boiler belongs to. On Warmflow Agentis and similar combi or system gas/oil boilers, E04 signals that the central heating circuit pressure has dropped below the minimum safe operating level — typically below 1 bar. The boiler shuts itself down as a precaution until adequate pressure is restored. On certain Warmflow external oil-fired boilers, E04 instead indicates that the flue thermofuse has tripped. This thermal safety device sits in the flue or exhaust pathway and cuts the boiler when it detects dangerously high flue temperatures, or when power was interrupted after a previous lockout. Always check your specific model's handbook first, as the meaning differs between ranges.

Causes, fixes & costs for Warmflow E04 → · All Warmflow codes

Frequently asked questions

Is the E04 fault code the same on every boiler?

No. E04 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 E04 fault myself?

On some brands there are safe homeowner checks first — typically system pressure, the condensate pipe in cold weather, and a single reset. Where the code is a simple pressure or reset condition, that may be all it needs. Never open the boiler casing yourself.

Who should I call about a E04 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.