E23 Boiler Fault Code: What It Means on Every Brand

The E23 fault code appears on 4 boiler brands — Alpha, Biasi, Keston and Ravenheat — 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 E23 means	Severity	DIY?
AAlpha	The E23 code on an Alpha boiler indicates that the control board has detected a problem with the heating return sensor (also called the return NTC thermistor). Full guide →	Lockout — engineer likely	Some checks
BBiasi	The E23 code on a Biasi boiler indicates that the PCB (printed circuit board) is attempting to communicate with the gas valve modulator coil — the component responsible for controlling the… Full guide →	Lockout — engineer likely	No
KKeston	The E23 fault code on a Keston boiler signals a water filling error. Full guide →	Lockout — engineer likely	Some checks
RRavenheat	The E23 code on a Ravenheat boiler signals a problem with the flue sensor — the component responsible for monitoring the temperature of exhaust gases as they leave the appliance. Full guide →	Lockout — engineer likely	No

E23 brand by brand

Alpha E23

The E23 code on an Alpha boiler indicates that the control board has detected a problem with the heating return sensor (also called the return NTC thermistor). This thermistor monitors the temperature of water coming back into the boiler from your central heating circuit, communicating that data to the PCB via changes in electrical resistance. When the reading falls outside expected parameters — or disappears entirely — the boiler locks out as a safety precaution and will not restart until the underlying problem is resolved. Hot water and central heating are both unavailable while this fault is active.

Causes, fixes & costs for Alpha E23 → · All Alpha codes

Biasi E23

The E23 code on a Biasi boiler indicates that the PCB (printed circuit board) is attempting to communicate with the gas valve modulator coil — the component responsible for controlling the rate of gas flow and therefore the flame size — but is receiving no response. In Biasi's own service documentation this is logged as 'E23: gas valve modulator disconnected'. The boiler will typically lock out as a safety precaution. Because this fault sits within the E20–E23 group (covering defective electromagnetic solenoid conditions), the underlying issue could be a disconnected wiring plug, a failed modulator coil, a faulty gas valve assembly, or in some cases a PCB that is no longer driving the modulator correctly. A Gas Safe registered engineer is needed to pinpoint which component is at fault.

Causes, fixes & costs for Biasi E23 → · All Biasi codes

Keston E23

The E23 fault code on a Keston boiler signals a water filling error. In plain terms, the boiler has detected a problem with the process of maintaining correct system water pressure, and the primary component to investigate is the water pressure sensor. When E23 appears, the boiler enters lockout and stops operating entirely — it will not restart on its own. You must press the Reset button once the underlying issue has been addressed. Simply switching the boiler off and on at the mains will not clear the lockout. E23 sits within Keston's broader fault hierarchy, sitting just below E24 (which flags water pressure loss four or more times within 24 hours), making it an important early warning that pressure management in your system needs attention.

Causes, fixes & costs for Keston E23 → · All Keston codes

Ravenheat E23

The E23 code on a Ravenheat boiler signals a problem with the flue sensor — the component responsible for monitoring the temperature of exhaust gases as they leave the appliance. When the boiler's control board receives a reading from this sensor that falls outside the expected range, or receives no signal at all, it shuts the boiler down as a precaution. In some Ravenheat models this circuit also incorporates a flue fusible link (a one-shot thermal safety device wired in series with the water pressure switch), which trips permanently if the flue reaches a dangerously high temperature. Whether the root cause is a physically damaged sensor, an electrical fault in the sensor circuit, or a genuine flue overheating event, the boiler will lock out and display E23 until the underlying problem is resolved.

Causes, fixes & costs for Ravenheat E23 → · All Ravenheat codes

Frequently asked questions

Is the E23 fault code the same on every boiler?

Broadly, yes — on Alpha, Biasi, Keston and Ravenheat the E23 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 E23 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 E23 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.