Keston E35 Fault Code: Causes, Fixes & Repair Costs
What does the Keston E35 fault code mean?
The E35 code appears when your Keston boiler detects a problem with the incoming electrical supply and locks out as a safety measure. The exact definition varies slightly by model: on the C36 Combi and C36P Combi, E35 means the mains voltage has dropped below 180V; on older Keston twin-flue models, it indicates the mains frequency is outside the expected 50Hz tolerance. In both cases the boiler has sensed something abnormal about the electricity reaching it — either the voltage is too low or the frequency is off — and has stopped operating until the issue is resolved. This is an electrical fault rather than a gas or heating-component fault, which means a qualified electrician may need to be involved alongside a Gas Safe 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
- Low incoming mains voltage Common
The voltage arriving at the boiler terminals has fallen below the 180V threshold the boiler requires to operate safely. This can happen during periods of high local demand on the electricity network, during brown-out conditions, or if there is a fault on the Distribution Network Operator's (DNO) infrastructure feeding the property.
- Faulty or deteriorating wiring to the boiler Common
A loose connection at the fused spur, a damaged supply cable, or an undersized wire running to the boiler can cause a measurable voltage drop at the appliance even when the mains supply elsewhere in the property appears normal. Connections that have worked loose over time are a surprisingly frequent culprit.
- Failing isolator switch or fused spur Sometimes
The switched fused connection unit (fused spur) that feeds the boiler can develop internal faults — a failing fuse holder, corroded contacts, or a faulty switch — that cause the voltage delivered to the boiler to fluctuate or drop below 180V even if the consumer unit is supplying the correct voltage.
- Mains frequency deviation Sometimes
Keston boilers require a stable 50Hz mains frequency. Significant deviation from this — which can occur after certain electrical work on the property's supply, or in rare cases from power conditioning equipment — can trigger E35 on older Keston twin-flue models in particular.
- PCB misreading the supply Rare
If the mains voltage and frequency both test within normal limits but E35 persists, the boiler's control board itself may be at fault — either incorrectly sensing the supply or suffering from an internal component failure. This is a less common cause but must be ruled out by an engineer once the electrical supply has been confirmed good.
How to fix it
- Switch the boiler off at its isolator or fused spur, wait 30 seconds, then switch it back on DIY safe
A brief power cycle can clear minor electronic glitches. Leave the boiler off for a full 30 seconds to allow the control board to fully reset before restoring power.
- Check that the isolator switch or fused spur supplying the boiler is fully on and visually undamaged DIY safe
Look for any signs of scorching, a tripped fuse, or a switch that appears partially off. Do not attempt to open or repair the unit yourself — if it looks damaged, leave it for a qualified electrician.
- Check whether other high-demand appliances are running on the same circuit at the same time DIY safe
Electric showers, ovens, and other high-draw appliances running simultaneously can drag the local voltage down. Try turning other heavy appliances off and then attempt a boiler reset to see whether E35 clears.
- Press the boiler reset button (no more than two or three times) DIY safe
If you believe the underlying cause may have resolved — for example the grid voltage has recovered — press reset to attempt to restart the boiler. If E35 returns immediately or after a short time, stop resetting and call an engineer. Repeated resetting will not fix a genuine electrical supply fault.
- Call a Gas Safe registered engineer to test the mains voltage at the boiler terminals Gas Safe engineer
An engineer needs to measure the actual voltage arriving at the boiler with test equipment. If the voltage is confirmed below 180V, they can advise whether the fault lies in the boiler's own wiring, the fused spur, or the wider property supply. Keston boilers — especially the twin-flue range — are considered complex appliances, so ask when booking whether the engineer has experience with Keston.
- If the supply voltage is confirmed low, engage a Part P registered electrician to investigate the property wiring and consumer unit Gas Safe engineer
Where voltage drop is traced back to the property's wiring or consumer unit rather than the boiler's own cable, a Part P electrician must carry out the remedial work. If the issue appears to originate with the DNO's network rather than the property's installation, the engineer or electrician can help you raise a fault report with your network operator.
- If the mains supply tests normal but E35 persists, have the PCB tested and replaced if necessary Gas Safe engineer
A Gas Safe engineer experienced with Keston boilers can assess whether the control board is falsely detecting a supply fault. PCB replacement on a Keston is a specialist job and the part itself is expensive, so get a written quote before authorising the work.
Parts you may need
- Keston C36 PCB / control board · from £320
- Switched fused connection unit (fused spur) · from £15
- Boiler supply cable (heat-resistant flex, per metre) · from £8
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–£350, depending on the underlying cause.
Frequently asked questions
Can I fix the Keston E35 fault myself?
The DIY steps are limited to a power cycle, checking the isolator switch visually, and pressing reset a couple of times. Beyond that, the fault involves the mains electrical supply, which needs to be tested by a Gas Safe engineer and potentially a Part P electrician — neither role is suitable for a homeowner to take on. Do not attempt to open the boiler or test wiring yourself.
Could my electricity supply actually be the problem, not the boiler?
Yes — and this is quite common with E35. The boiler is simply reporting what it senses at its terminals. If the DNO's network is delivering low voltage to your area, or if there is a fault in your property's consumer unit or supply cable, the boiler will flag E35 even though nothing inside the boiler is broken. In that scenario, the repair cost is largely an electrical job and the boiler diagnostic visit may be all that is needed on the heating side.
What does E35 mean on older Keston twin-flue boilers versus the C36 Combi?
On the Keston C36 Combi and C36P Combi, E35 specifically means the mains voltage has dropped below 180V. On the older Keston twin-flue range, the same code indicates the mains frequency is outside the expected 50Hz tolerance rather than a voltage issue. The practical response is the same — a Gas Safe engineer testing the supply — but it is worth mentioning which model you have when you call, as the engineer's diagnostic approach may differ slightly.
How much does it cost to repair a Keston E35 fault, and could it be more expensive?
Most people with an E35 fault end up paying somewhere between £100 and £350, covering a diagnostic visit and any straightforward wiring or isolator work. If the problem turns out to be a DNO network issue, costs may be limited to the engineer's visit fee alone. However, if the PCB is found to be faulty, replacement on a Keston boiler typically costs £500 or more including parts and labour — that scenario is less common but worth budgeting for if the supply checks out fine. For older Keston boilers (10 or more years old), it is worth getting a replacement quote alongside the repair quote before committing.