E43 Boiler Fault Code: What It Means on Every Brand

The E43 fault code appears on 4 brands — Alpha, Keston, Ravenheat and Remeha — 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 E43 means	Severity	DIY?
AAlpha	The E43 fault code on Alpha boilers (including InTec, E-Tec, and Eco2 models) has two well-documented interpretations that vary by model. Full guide →	Lockout — engineer likely	Some checks
KKeston	The E43 fault code indicates that the Keston boiler's control system has detected a short circuit in the return pipe temperature sensor (thermistor). Full guide →	Lockout — engineer likely	No
RRavenheat	The E43 code on a Ravenheat boiler sits within the E42–E44 fault group, which collectively point to a 'different temperature points' issue — in other words, the boiler's control board is… Full guide →	Lockout — engineer likely	No
RRemeha	The E43 fault code on a Remeha boiler signals that one or more internal automation parameters have moved outside the range the boiler's control system is programmed to accept. Full guide →	Lockout — engineer likely	Some checks

E43 brand by brand

Alpha E43

The E43 fault code on Alpha boilers (including InTec, E-Tec, and Eco2 models) has two well-documented interpretations that vary by model. The first — and more common — is a loss of flame or ignition failure: the boiler fired but could not establish or maintain a stable flame signal, so it shut down as a safety measure. The second interpretation, seen more often on certain control board variants, is a communication failure between the main PCB and one or more components. In both cases the boiler locks out and will not restart until the underlying problem is resolved. Always check your specific model's manual, as Alpha's range spans several generations with slightly different behaviour for the same code.

Causes, fixes & costs for Alpha E43 → · All Alpha codes

Keston E43

The E43 fault code indicates that the Keston boiler's control system has detected a short circuit in the return pipe temperature sensor (thermistor). This sensor monitors the temperature of water coming back into the boiler after it has passed around your radiators and heating circuit. Working alongside the flow sensor, it helps the boiler regulate efficiency and keep temperature differentials within safe limits. A short circuit — as opposed to an open circuit — means the sensor circuit is showing near-zero resistance, which the boiler interprets as an implausible reading. Depending on severity, the boiler may temporarily block operation and attempt to restart automatically, displaying the block code alternately. If the fault is persistent, it will lock out completely and require a manual reset. Because this fault involves internal electrical diagnosis, it is not suitable for DIY repair and requires a Gas Safe registered engineer.

Causes, fixes & costs for Keston E43 → · All Keston codes

Ravenheat E43

The E43 code on a Ravenheat boiler sits within the E42–E44 fault group, which collectively point to a 'different temperature points' issue — in other words, the boiler's control board is receiving conflicting or out-of-range temperature readings from one or more of its NTC thermistors (temperature sensors). In practice, E43 is closely associated with flame detection or ignition failure, where the boiler loses its flame signal during operation, sometimes repeatedly. The two main NTC sensors — one measuring the flow (outgoing hot water) temperature and one measuring the return (cooler incoming water) temperature — constantly feed data to the PCB. If either sensor sends erratic, implausible, or out-of-range values, the PCB triggers E43 as a safety lockout. Note that Ravenheat uses slightly different code sets across its RSF, CSI, and LS model ranges, so always check your specific model's documentation if the behaviour seems unusual.

Causes, fixes & costs for Ravenheat E43 → · All Ravenheat codes

Remeha E43

The E43 fault code on a Remeha boiler signals that one or more internal automation parameters have moved outside the range the boiler's control system is programmed to accept. In plain terms, the boiler's brain has detected that a setting or operating value is out of bounds and has locked the boiler out as a precaution. This prevents potential damage to components and keeps the household safe. The fault is most commonly linked to a corrupted or incorrectly configured parameter set rather than a failed mechanical part, so a reset — or a quick engineer visit to re-programme the settings — is often all that is needed.

Causes, fixes & costs for Remeha E43 → · All Remeha codes

Frequently asked questions

Is the E43 fault code the same on every boiler?

No. E43 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 E43 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 E43 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.