Keston E05 Fault Code: Fan Fault or Boiler Overheat
The E05 code is a lockout fault on Keston boilers, but its precise meaning varies by model. On the Keston C36 combi, E05 is most commonly associated with a fan fault — the boiler's printed circuit board cannot confirm that the fan is operating correctly, so it shuts down to prevent dangerous flue gases from building up. On some C36 variants, however, E05 is recorded as a boiler overheat fault, where an internal safety thermostat has tripped because the boiler reached an unsafe temperature. Both interpretations are credible and supported by specialist sources, so both are addressed in this guide. In either case, the boiler will lock out and refuse to restart until the reset button is pressed.
General guidance only — not a substitute for professional advice. 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.
What does the Keston E05 fault code mean?
The E05 code is a lockout fault on Keston boilers, but its precise meaning varies by model. On the Keston C36 combi, E05 is most commonly associated with a fan fault — the boiler's printed circuit board cannot confirm that the fan is operating correctly, so it shuts down to prevent dangerous flue gases from building up. On some C36 variants, however, E05 is recorded as a boiler overheat fault, where an internal safety thermostat has tripped because the boiler reached an unsafe temperature. Both interpretations are credible and supported by specialist sources, so both are addressed in this guide. In either case, the boiler will lock out and refuse to restart until the reset button is pressed.
Common causes
- Fan motor failure or excessive wear Common
The fan motor windings can burn out over time, or the bearings deteriorate to the point where the fan either stops completely or cannot reach the speed the PCB expects. Without confirmation of correct fan rotation, the boiler locks out as a safety precaution.
- Blocked venturi or flue passage Common
Spiders and insects are notorious for nesting inside the venturi — a small plastic tube that senses airflow. Even a partial blockage can prevent the fan from generating enough draught, causing the air pressure switch to signal a fault. Debris in the flue itself can have the same effect.
- Overheating due to a seized or failing pump Common
If E05 is being thrown as an overheat code, a pump that is running too slowly or has seized altogether is the most frequent culprit. Without adequate circulation, heat builds up rapidly in the heat exchanger and the overheat thermostat trips.
- Air pressure switch fault Sometimes
The air pressure switch monitors the draught created by the fan. If the switch itself has failed — or the small rubber tube connecting it to the venturi has cracked or become disconnected — it may report a fan fault even when the fan is physically fine.
- Sludge or scale blocking the heat exchanger Sometimes
In systems with poor water quality or no inhibitor, magnetite sludge and limescale can restrict flow through the heat exchanger. This causes localised overheating and can trigger the overheat thermostat, producing an E05 lockout on susceptible C36 models.
- PCB not reading fan speed signal correctly Rare
The fan sends a tachometric signal back to the PCB so it can confirm correct rotation speed. If the wiring connector is loose, corroded, or the PCB itself is developing a fault, it may misread or lose this signal entirely — locking out the boiler even though the fan is mechanically sound.
- Air trapped in the heating circuit Rare
Significant air pockets in the system reduce the efficiency of heat transfer and can cause the boiler to overheat in short bursts, potentially tripping the overheat thermostat and generating an E05 on overheat-configured models.
How to fix it
- Check your gas supply is working DIY safe
Before assuming the boiler has a serious fault, confirm that other gas appliances in your home — such as a gas hob — are working normally. If there is no gas supply to the property, contact your gas supplier rather than a boiler engineer.
- Attempt a single boiler reset DIY safe
Press and hold the reset button on the boiler's control panel as described in your user guide. Wait for the boiler to complete its start-up sequence. If the E05 code returns immediately or within a short time, do not keep resetting — repeated resets on a fan or overheat fault can mask a genuine safety problem. Limit yourself to two or three attempts total.
- Check the system pressure gauge DIY safe
Locate the pressure gauge on the boiler front panel. It should read between 1.0 and 1.5 bar when the system is cold. If it reads below 0.8 bar, you can top it up via the filling loop (refer to your boiler's user guide for the correct procedure). Very low pressure can contribute to poor circulation and overheating, though it is unlikely to be the sole cause of E05.
- Look for obvious external blockages at the flue terminal DIY safe
Go outside and check that the flue outlet — usually a white plastic terminal on an external wall or roof — is not visibly obstructed by a bird's nest, leaves, or ice. Do not attempt to dismantle the flue; simply note what you see and report it to the engineer.
- Call a Gas Safe registered engineer with Keston experience Gas Safe engineer
E05 requires professional diagnosis regardless of whether it is a fan fault or an overheat condition. Keston boilers feature a twin-flue design and a modulating pump, which makes them more complex than many standard combi boilers — not all engineers are familiar with them, so it is worth asking specifically whether the engineer has worked on Keston appliances before booking. The engineer will: test the fan motor for correct voltage and rotational speed; inspect and clear the venturi and flue of any debris or blockages; check the air pressure switch and its connecting tubing; test pump operation and flow rates if an overheat fault is suspected; assess the heat exchanger for sludge or scale; and replace whichever component has failed.
Parts you may need
- Boiler fan / flue fan assembly (C36 compatible) · from £145
- Air pressure switch · from £35
- Water circulating pump · from £85
- Overheat thermostat / high-limit stat · from £25
- Venturi tube / air sensing tube · from £18
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 £220–£380, depending on the underlying cause.
Frequently asked questions
Can I reset a Keston E05 fault myself?
Yes, you can press the reset button yourself — and it is worth trying once or twice. However, if E05 comes back after resetting, stop there. Repeatedly resetting a boiler with a fan or overheat fault does not fix the underlying problem and can occasionally make diagnosis harder. You will need a Gas Safe engineer to identify and repair the root cause.
Is Keston E05 always a fan problem, or could it be something else?
It depends on your exact model. On the Keston C36 combi, E05 is most frequently linked to a fan fault — either a failing motor, blocked venturi, or a faulty air pressure switch. On some C36 configurations, E05 is recorded as an overheat fault, which points instead to pump issues, sludge in the heat exchanger, or trapped air in the system. A qualified engineer will be able to distinguish between the two by testing each component in turn.
How much does it cost to repair a Keston E05 fault in the UK?
For the most common cause — fan replacement — expect to pay roughly £220 to £320 all-in for parts and labour. If the fault turns out to be overheat-related, the cost depends on what has failed: an overheat thermostat is relatively inexpensive (around £80–£130 fitted), while a pump replacement typically runs £150–£350. In rare cases where the heat exchanger is blocked beyond cleaning or has cracked, costs can rise considerably higher — if your boiler is already over 12–15 years old, it may be worth getting a replacement quote at the same time.
Why do Keston boilers need a specialist engineer?
Keston boilers use a twin-flue arrangement (separate pipes for air intake and exhaust) and a modulating pump, which differs from the layout most engineers encounter on mainstream brands. This means fault diagnosis and parts sourcing can take longer, and an engineer unfamiliar with the design may misdiagnose the problem. Always ask whether the engineer has previous Keston experience before booking — many heating firms do, but it is worth confirming.