Keston E43 Fault Code: Causes, Fixes & Repair Costs
What does the Keston E43 fault code mean?
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.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
Common causes
- Loose or corroded wiring connector at the return sensor Common
The most frequent trigger for E43. The connector plug at the return thermistor can work loose over time due to vibration, or the terminals can corrode — particularly in older boilers or those in damp utility rooms. Either condition can create an unintended short circuit in the signal wiring, causing the control board to log E43.
- Failed return thermistor Common
The NTC thermistor itself can fail internally, causing its resistance to drop to near zero and mimic a short circuit. Thermistors are relatively inexpensive components, but when they fail in this way they produce exactly the signal E43 is designed to detect. Replacing the sensor is usually the first practical repair after wiring has been ruled out.
- Moisture or corrosion in the wiring harness Sometimes
Water ingress around the sensor harness — from a minor internal leak, condensation, or a nearby pipe drip — can track along the wiring and cause a short between conductors. This is more common on boilers that have not been serviced regularly or that are installed in poorly ventilated spaces.
- PCB misreading the sensor signal Rare
If the wiring and thermistor both test correctly, the fault may originate on the printed circuit board itself. The PCB interprets the resistance signal from the return thermistor, and internal component failure on the board can cause it to report a short circuit even when none exists in the sensor circuit. This is the least common cause but can be the most costly to resolve.
How to fix it
- Check whether the boiler has locked out or is in a temporary block DIY safe
Look at the boiler display. If E43 is showing alternately with another code, the boiler is in a timed block and may attempt to restart automatically. If it has locked out and the display shows only E43, the boiler will not fire until it is reset.
- Attempt a single reset DIY safe
Press and hold the reset button on the boiler fascia as described in your user manual — typically for around 3 seconds. Allow the boiler a couple of minutes to attempt ignition. If E43 returns immediately or within a short time, do not keep resetting repeatedly, as this will not clear an underlying electrical fault and may mask important diagnostic information for your engineer.
- Contact a Gas Safe registered engineer with Keston experience Gas Safe engineer
Keston boilers use a twin-flue design with a modulating pump, which not all engineers are familiar with. It is worth specifically asking whether the engineer has worked on Keston appliances before booking. You can use the engineer finder at keston.co.uk or call the Keston technical helpline for a referral to a locally approved engineer.
- Engineer to inspect and reseat wiring connectors at the return sensor Gas Safe engineer
The engineer's first step should be to locate the return thermistor and carefully inspect the connector plug and wiring for signs of looseness, corrosion, or chafing. Reseating or replacing a corroded connector can resolve E43 at minimal cost and is the quickest fix to rule out.
- Engineer to test the return thermistor resistance Gas Safe engineer
Using a multimeter, the engineer will measure the thermistor's resistance and compare it against the expected NTC resistance curve for the water temperature at the time of testing. A short circuit will typically show resistance close to zero ohms. If the thermistor fails this test, it should be replaced.
- Engineer to replace the return thermistor if faulty Gas Safe engineer
The return thermistor is an inexpensive part — typically £10–£30 — and replacement is straightforward for an experienced engineer. After fitting the new sensor, the engineer should reset the boiler and confirm that E43 does not return during a full heat cycle.
- Engineer to investigate the PCB if E43 persists after sensor replacement Gas Safe engineer
If a new thermistor and sound wiring do not clear the fault, the PCB must be assessed. The engineer may be able to perform a parameter reset on the board, but if the board has an internal failure it will need replacing. PCB replacement on a Keston is a significant cost — see the FAQ below — so an engineer should confirm the diagnosis carefully before committing to this repair.
Parts you may need
- Return pipe NTC thermistor (Keston compatible) · from £20
- Sensor connector/wiring harness · from £15
- Keston PCB (control board) · from £320
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £100–£250, depending on the underlying cause.
Frequently asked questions
Can I reset the boiler myself when E43 appears?
Yes, a single reset attempt is safe for a homeowner — press and hold the reset button as described in your user manual. However, if E43 returns promptly after resetting, stop there. Repeated resets will not fix an electrical fault in the sensor circuit and may make fault diagnosis harder for your engineer. Call a Gas Safe engineer if the code keeps coming back.
How much does it cost to fix a Keston E43 fault?
If the fix turns out to be a wiring connector issue or a thermistor replacement, most homeowners pay between £100 and £250 once you include the call-out, labour, and the inexpensive part. The thermistor itself typically costs only £10–£30. If the PCB is at fault, costs rise sharply — a replacement Keston PCB can be £280–£400 for the part alone, plus labour, pushing the total to £450 or more. A competent engineer should be able to tell you which component is at fault before ordering parts.
Why does it matter if an engineer has Keston experience?
Keston boilers are less common than mainstream brands and use a distinctive twin-flue configuration with a modulating pump. Engineers unfamiliar with the design can spend longer on diagnosis and may not have the right reference data for thermistor resistance values or wiring layouts. Keston's own technical helpline (available via keston.co.uk) can refer you to engineers they know are comfortable working on their appliances, which often means a faster and more accurate repair.
What is the difference between a short circuit fault (E43) and an open circuit fault on the return sensor?
An open circuit means the electrical connection to the sensor is completely broken — a wire has snapped or a connector has fully disconnected — giving an infinitely high resistance reading. A short circuit, as indicated by E43, means there is an unintended direct connection somewhere in the circuit, giving near-zero resistance. Both prevent the boiler from getting a valid temperature reading, but they have different causes and are tested differently. Your engineer will use a multimeter to distinguish between the two and identify whether the fault is in the wiring, the sensor itself, or the PCB.