Keston E45 Fault Code: Causes, Fixes & Repair Costs
What does the Keston E45 fault code mean?
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.
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
- Damaged or pinched wiring harness Common
The cable running from the flue sensor back to the control box can become chafed, trapped, or corroded over time. If two conductors within the loom touch each other, a short circuit is created and E45 is triggered. This is a frequent finding during diagnosis.
- Faulty flue sensor (NTC thermistor) Common
The thermistor itself can fail internally, producing a near-zero resistance reading even when the wiring is intact. A Gas Safe engineer can confirm this quickly by measuring resistance with a multimeter and cross-referencing against the manufacturer's temperature-resistance table.
- Loose, corroded, or poorly seated connector plugs Sometimes
Keston's own guidance specifically highlights checking the sensor connections as the first action. Connector plugs that have corroded, worked loose, or been incorrectly reseated can bridge the circuit and mimic a short circuit.
- Moisture or condensation ingress Sometimes
Water tracking into the sensor housing or its connector — whether from a nearby condensate leak, general condensation inside the casing, or a poorly sealed flue joint — can bridge the circuit and produce a false short-circuit reading.
- PCB fault Rare
If the wiring loom and the sensor both test as healthy, the fault may originate on the main control board itself. The PCB may have developed an internal fault on the sensor monitoring circuit, or lost its stored parameters. This is the least common cause but the most costly to fix.
How to fix it
- Check the boiler display and note the exact code shown DIY safe
Confirm the display reads E45 rather than the related E46 (open circuit) code. This matters because the two faults have different root causes, and telling an engineer which code appeared helps them prepare the right parts and tests before visiting.
- Press the reset button once to attempt to clear the lockout DIY safe
Locate the reset button on the boiler fascia — on most Keston models it is labelled and requires a firm press for a few seconds. If the boiler fires and runs normally, monitor it closely. If E45 returns within a short period, do not keep resetting; repeated lockouts on the same fault indicate a genuine component problem that needs professional attention.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in your home — hob, gas fire — are functioning normally. If they are not, contact your gas supplier rather than an engineer, as the issue lies with the supply rather than the boiler.
- Do not attempt to access the flue sensor, wiring, or PCB yourself Gas Safe engineer
The flue sensor, its wiring loom, and the control board are all internal components. Accessing them requires removing boiler casing and working near gas-carrying parts. This work must only be carried out by a Gas Safe registered engineer. There is no homeowner-safe shortcut here.
- Contact a Gas Safe registered engineer with Keston experience Gas Safe engineer
Keston boilers use a twin-flue design and modulating pump that not every engineer is familiar with. When booking, ask whether the engineer has worked on Keston boilers before. The engineer will trace the wiring loom for damage, clean and reseat the connector plugs, measure the thermistor resistance with a multimeter, and replace the sensor or PCB as required. Ask for a fixed-price written quote before work begins.
Parts you may need
- Flue NTC thermistor (flue sensor) · from £35
- Wiring loom / sensor cable harness · from £45
- Connector plug set · from £15
- Main PCB (control board) · from £280
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 £150–£350, depending on the underlying cause.
Frequently asked questions
Can I fix Keston E45 myself?
No. While you can safely press the reset button once or twice to see whether the fault clears, any work on the flue sensor, wiring, or PCB must be carried out by a Gas Safe registered engineer. Accessing these components involves removing the boiler casing and working in close proximity to gas-carrying parts, which is illegal and dangerous for anyone who is not Gas Safe registered.
What is the difference between Keston fault codes E45 and E46?
Both codes relate to the flue sensor circuit, but they point to opposite electrical faults. E45 means a short circuit — the resistance in the sensor circuit has collapsed to near-zero, as if the two conductors are touching. E46 means an open circuit — the connection is broken entirely, so no signal reaches the PCB at all. The diagnostic approach is similar (check wiring, connectors, and the sensor itself), but telling your engineer which code appeared helps them narrow down the fault more quickly.
How much does it cost to fix Keston E45 in the UK?
For the most common repairs — reconnecting or replacing the flue sensor and its wiring — most homeowners pay between £150 and £350 including parts and labour. The sensor itself is an inexpensive part; the bulk of the cost is the engineer's callout and diagnostic time. If the fault turns out to be a failed PCB (main circuit board), costs rise significantly — typically £500 or more — because the board itself is expensive. If your boiler is over 10–12 years old and a PCB is needed, it may be worth comparing the repair cost against a new boiler installation.
Will my Keston boiler restart on its own after an E45 fault?
Not automatically. Keston boilers require you to press the reset button on the fascia before they will attempt to fire again after a lockout. Simply switching the power off and back on will not clear the fault. If the boiler locks out again shortly after resetting, stop resetting it and call a Gas Safe engineer — repeated resets without fixing the underlying cause can mask the fault and, in some cases, cause further damage.