E03 Boiler Fault Code: What It Means on Every Brand

The E03 fault code appears on 5 boiler brands — Biasi, Johnson & Starley, Keston, Ravenheat and Warmflow — 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 5 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 E03 means	Severity	DIY?
BBiasi	The E03 (sometimes displayed as ER03) fault code on a Biasi boiler signals a generic lockout. Full guide →	Lockout — engineer likely	Some checks
JSJohnson & Starley	E03 on a Johnson & Starley unit is the overheat lockout: a temperature inside the appliance exceeded its safety limit and the overheat protection shut it down. Full guide →	Lockout — engineer likely	Some checks
KKeston	The E03 code on Keston boilers does not have a single universal meaning — its interpretation depends on which model you own. Full guide →	Lockout — engineer likely	No
RRavenheat	The E03 code on a Ravenheat boiler indicates that the PCB has detected a problem with the flow sensor — the small NTC thermistor that measures the temperature of water leaving the heat… Full guide →	Lockout — engineer likely	No
WWarmflow	The E03 code appears on Warmflow boilers when the flue thermofuse has tripped. Full guide →	Lockout — engineer likely	No

E03 brand by brand

Biasi E03

The E03 (sometimes displayed as ER03) fault code on a Biasi boiler signals a generic lockout. Unlike more targeted Biasi fault codes — for example, ER05 which points specifically at the fan circuit — E03 is a catch-all lockout: the boiler's PCB has detected a reading outside of acceptable limits and shut the appliance down as a safety measure. Because it is non-specific, E03 can be triggered by a range of underlying problems, from a frozen condensate pipe to a faulty air pressure switch, a mains voltage or earthing issue, or a struggling pump. The boiler will not restart until either the root cause is resolved or, in some cases, until a manual reset is performed.

Causes, fixes & costs for Biasi E03 → · All Biasi codes

Johnson & Starley E03

E03 on a Johnson & Starley unit is the overheat lockout: a temperature inside the appliance exceeded its safety limit and the overheat protection shut it down. On a warm-air heater that generally means the heat exchanger ran too hot — which happens when heat is being produced faster than the airflow is carrying it away. Restricted return air, blocked filters or outlets, a failing circulation fan, or a control fault driving excess output are the practical causes; on water-carrying models, poor water circulation does the same on that side. Overheat lockouts must not be reset repeatedly: the protection is doing its job, and the airflow or circulation problem behind it needs finding.

Causes, fixes & costs for Johnson & Starley E03 → · All Johnson & Starley codes

Keston E03

The E03 code on Keston boilers does not have a single universal meaning — its interpretation depends on which model you own. On the C36 Combi, E03 signals a 'false flame' condition, meaning the boiler's control board is detecting the presence of a flame before the ignition sequence has actually been triggered. On C40 and C55 Combi models, the code carries a similar meaning: flame detected when none is expected. On some older Keston Combi boilers (including certain Qudos variants), E03 points specifically to a faulty gas valve. A further variant exists on some C36 firmware versions, where E03 indicates an overheat lockout — the boiler has shut itself off as a safety measure because internal temperatures climbed too high. In rare cases on C40/C55 units, a flashing E03 (sometimes appearing alongside E37) can indicate a pressure lockout. Always check your specific model's documentation to confirm which definition applies, and use the causes below to narrow down what is most likely happening on your boiler.

Causes, fixes & costs for Keston E03 → · All Keston codes

Ravenheat E03

The E03 code on a Ravenheat boiler indicates that the PCB has detected a problem with the flow sensor — the small NTC thermistor that measures the temperature of water leaving the heat exchanger. Because the PCB relies on accurate readings from this sensor to control burner activity safely, a damaged or unreadable sensor causes the boiler to lock out as a precaution. The fault can be caused by a physically failed thermistor, damaged wiring, or limescale coating the probe. One important note for Ravenheat owners: Ravenheat Manufacturing Limited was dissolved in 2025, meaning spare parts are increasingly difficult to source. Before committing to a repair, consider whether the cost makes sense versus replacing the boiler entirely — especially on units over ten years old.

Causes, fixes & costs for Ravenheat E03 → · All Ravenheat codes

Warmflow E03

The E03 code appears on Warmflow boilers when the flue thermofuse has tripped. The thermofuse is a dedicated safety component mounted on the flue — the pipe that carries waste combustion gases out of the boiler. Its job is to detect dangerously high flue temperatures and cut boiler operation before any damage can occur. Once triggered, the boiler goes into a hard lockout and will not restart until the underlying cause has been identified and resolved. Because this fault is directly tied to combustion safety, it must be investigated by a qualified engineer — Gas Safe registered for gas models, or OFTEC registered for oil-fired Warmflow appliances.

Causes, fixes & costs for Warmflow E03 → · All Warmflow codes

Frequently asked questions

Is the E03 fault code the same on every boiler?

Broadly, yes — on Biasi, Johnson & Starley, Keston, Ravenheat and Warmflow the E03 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 E03 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 E03 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.