E45 Boiler Fault Code: What It Means on Every Brand

The E45 fault code appears on 3 boiler brands — Alpha, Keston and Remeha — 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 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 E45 means	Severity	DIY?
AAlpha	The E45 fault code on Alpha boilers signals a 'High Delta T' condition — the boiler has detected an unexpectedly large temperature difference between the flow (water leaving the boiler)… Full guide →	Lockout — engineer likely	Some checks
KKeston	The E45 code on a Keston boiler indicates that the control board has detected a short circuit in the flue sensor circuit. Full guide →	Lockout — engineer likely	No
RRemeha	The E45 code appears on Remeha boilers when the internal control unit (PCB) has lost its programmed settings and fallen back to factory default parameters. Full guide →	Lockout — engineer likely	No

E45 brand by brand

Alpha E45

The E45 fault code on Alpha boilers signals a 'High Delta T' condition — the boiler has detected an unexpectedly large temperature difference between the flow (water leaving the boiler) and the return (water coming back from the heating system). In straightforward terms, the water being sent out is significantly hotter than what is returning, which tells the boiler that not enough water is circulating around the system. As a safety measure, the boiler limits burner output or locks out entirely to protect the heat exchanger from overheating. Once normal circulation is restored and the temperature differential falls back within acceptable limits, the boiler can resume regular operation.

Causes, fixes & costs for Alpha E45 → · All Alpha codes

Keston E45

The E45 code on a Keston boiler indicates that the control board has detected a short circuit in the flue sensor circuit. The flue sensor is a small NTC thermistor — a temperature-measuring component — fitted in the exhaust gas path. By monitoring changes in its electrical resistance, it feeds live temperature data back to the PCB. A short circuit means the resistance has dropped to near-zero, sending an implausible signal that the boiler cannot safely act on. Depending on severity, the boiler may block firing temporarily and retry, or lock out entirely until the reset button is pressed. Note that the related code E46 points to the opposite problem — an open circuit (broken connection) in the same sensor circuit rather than a short.

Causes, fixes & costs for Keston E45 → · All Keston codes

Remeha E45

The E45 code appears on Remeha boilers when the internal control unit (PCB) has lost its programmed settings and fallen back to factory default parameters. In this state the boiler can no longer run correctly, so it locks out and displays E45 to alert you that the brain of the appliance needs attention. The fault can arise either because the PCB itself has suffered a hardware failure, or because the settings stored on it have become corrupted – both outcomes lead to the same lockout. This is not a fault you can clear with a simple reset; the underlying control unit issue must be resolved first.

Causes, fixes & costs for Remeha E45 → · All Remeha codes

Frequently asked questions

Is the E45 fault code the same on every boiler?

Broadly, yes — on Alpha, Keston and Remeha the E45 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 E45 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 E45 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.