Keston E34 Fault Code: Causes, Fixes & Repair Costs
What does the Keston E34 fault code mean?
The E34 code has two distinct meanings depending on which Keston model you own — so the first step is always to confirm your exact model number. On the Keston C36 Combi, E34 indicates that the mains supply voltage feeding the boiler has fallen below 180V, pointing to an electrical supply issue rather than a fault with the boiler itself. On the Keston C36P Combi and similar newer models, E34 means the boiler has experienced a flame dropout — it either failed to establish a flame during ignition or lost the flame unexpectedly during operation. In both cases the boiler will lock out and refuse to restart until the fault is addressed and the reset button is pressed.
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
- Frozen or blocked condensate drain pipe (C36P flame dropout) Common
In cold weather the external condensate pipe — usually a white plastic pipe running to an outside drain — can freeze solid, causing the boiler to shut down. Keston twin-flue models are particularly prone to this because of their condensate routing. This is one of the most common triggers for an E34 flame dropout in winter.
- Low gas supply pressure (C36P flame dropout) Common
If the gas pressure at the boiler inlet falls below the required 20 mbar, the burner cannot sustain a stable flame and the boiler trips out. This can happen on cold mornings when demand across the gas network is high, or on LPG installations where the tank is running low.
- Mains supply voltage below 180V (C36 low voltage) Common
On the C36, a drop in incoming mains voltage — caused by a wider supply issue, a deteriorating fused spur, or ageing wiring to the boiler — triggers E34 directly. Other appliances on the same circuit may also behave erratically if this is the cause.
- Cracked or worn ignition electrode (C36P flame dropout) Sometimes
The ignition electrode generates the spark that lights the burner. Over time the ceramic casing can crack or the tip can erode, producing a weak or intermittent spark. The boiler attempts ignition, fails to detect a flame, and locks out on E34.
- Faulty gas valve (C36P flame dropout) Sometimes
The modulating gas valve controls fuel flow to the burner. If it sticks, fails to open fully, or develops an internal fault, the burner receives insufficient gas and the flame either fails to establish or extinguishes shortly after lighting.
- Burner degradation (C36P flame dropout) Sometimes
Keston boilers use a stainless steel burner that can distort or corrode after many years of high-temperature cycling. A deformed burner produces an uneven flame that the flame detection circuit may not recognise as stable, causing repeated dropouts.
- PCB (printed circuit board) fault Rare
On older Keston models the control board can develop faults that affect ignition sequencing or flame signal interpretation, causing spurious E34 lockouts even when the gas and electrical supplies are perfectly normal.
- Flue or air terminal obstruction (C36P flame dropout) Rare
A blocked flue outlet or contaminated air intake — caused by a bird's nest, debris, or ice build-up — disrupts combustion airflow and can cause the flame to become unstable or extinguish, particularly on twin-flue installations.
How to fix it
- Confirm your model number and match the fault definition DIY safe
Check the data plate on the front or side of the boiler casing. If you have a C36, E34 relates to mains voltage; if you have a C36P or similar, it relates to flame dropout. This determines which checks are relevant and what to tell an engineer.
- Check that the gas supply is on and other appliances are working DIY safe
Make sure the gas isolation valve behind the boiler is open and that other gas appliances in the property — a hob or gas fire — are working normally. If no gas appliances are functioning, contact your gas supplier before doing anything else.
- Inspect the condensate pipe for freezing (C36P only, during cold weather) DIY safe
Locate the white plastic condensate pipe that runs from the boiler to an external drain. If it feels solid or you can see ice around it, carefully pour warm (not boiling) water along the pipe until it flows freely. Never use a naked flame or boiling water. Once thawed, reset the boiler.
- Check the fused spur and mains supply (C36 low voltage variant) DIY safe
For the C36, check that the fused spur or switched connection unit supplying the boiler has not tripped and that the fuse inside the unit is intact. Also check whether other appliances on the same ring main or circuit are behaving normally, which may indicate a wider supply issue to report to your network operator.
- Reset the boiler (up to twice) DIY safe
Press and hold the reset button — marked with a spanner or flame symbol depending on the model — for 3 to 5 seconds until the boiler attempts to restart. If it locks out again immediately, do not reset more than twice in quick succession as repeated failed ignitions can cause unburnt gas to accumulate. If the fault persists after two resets, stop and call an engineer.
- Arrange a visit from a Gas Safe registered engineer with Keston experience Gas Safe engineer
Because E34 can involve ignition components, the gas valve, the PCB, or the incoming gas pressure, further diagnosis and repair must be carried out by a Gas Safe registered engineer. Keston boilers are considered complex appliances due to their twin-flue design and modulating pump, so it is worth specifically asking whether the engineer has worked on Keston or Ideal Heating twin-flue models before. The engineer will verify inlet gas pressure (should be 20 mbar at the valve), check ignition electrode condition and spark gap, test the gas valve, inspect the flue terminals, and confirm the incoming mains voltage is a stable 230V. Use the engineer finder at keston.co.uk or contact the Keston technical helpline for a recommended local specialist.
Parts you may need
- Ignition electrode assembly · from £35
- Ignition transformer · from £55
- Gas valve (modulating) · from £120
- Burner assembly · from £90
- PCB (control board) · from £220
- Fused spur / connection unit (electrical) · from £15
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 £120–£380, depending on the underlying cause.
Frequently asked questions
How do I know if my Keston E34 is the voltage fault or the flame dropout fault?
It depends entirely on your model. Check the data plate on the boiler casing — if it says C36 (without a P), E34 is a low mains voltage alert. If it says C36P or a similar later variant, E34 is a flame dropout fault. If you are unsure, your boiler's installation manual will list what each code means for that specific model, and Keston's technical helpline can confirm it over the phone.
Can I fix a frozen condensate pipe myself, and will that clear the E34?
Yes — thawing a frozen condensate pipe is one of the few boiler-related tasks a homeowner can safely carry out. Pour warm (not boiling) water over the external white plastic pipe until it runs freely, then reset the boiler. If E34 was triggered by a freeze, the boiler should restart normally. To prevent a recurrence, a Gas Safe engineer can lag the exposed pipe with foam insulation — a straightforward and inexpensive job.
My engineer is quoting over £500 to fix my Keston E34 — is it worth repairing?
It depends on the age and service history of the boiler. If the fault turns out to be the PCB — which can cost £300–£500 in parts alone — and the boiler is more than ten years old or has been poorly maintained, replacement may offer better long-term value than repair. Ask your engineer to give you a written quote for both the repair and their assessment of overall boiler condition before committing. Keston parts and technical support remain available through Ideal Heating, so parts availability is not yet a concern for most models.
Why do Keston boilers need a specialist engineer rather than any Gas Safe engineer?
Keston boilers use a twin-flue system (separate ducts for air intake and exhaust) combined with a modulating pump and high-efficiency heat exchanger. This design differs significantly from standard single-flue combi boilers, and fault diagnosis can be more involved. Not all Gas Safe engineers are familiar with Keston-specific wiring, flue configurations, or the correct commissioning procedures. Keston's own website at keston.co.uk maintains an engineer finder, and their technical helpline can recommend local engineers who have specific experience with these models.