Keston boilers might not be as common as Worcester or Vaillant, but thousands of British homes – especially new-build flats, apartments with long flue runs, and properties where a standard boiler simply won’t fit – rely on them every day.
The current Keston C35, C40, Qudos, QSpa, and the older twin-flue models are brilliant when they work, but when they flash an error code they can leave you completely in the dark because most generic “boiler fault” websites barely mention Keston.
This guide is written for every UK homeowner or landlord who’s staring at an E-number, L-number or just a flashing red light on their Keston right now.
How Keston Displays Faults
- Modern Keston C35 / C40 / C40P (2018 onwards): large digital display shows E** or L** codes
- Keston Qudos 28 / QSpa: small LED screen with E-codes
- Older twin-flue Keston Celsius / Combi: alternating temperature and fault number, or just flashing lights
All Keston boilers lock out with a solid or flashing red light when something serious happens.
Most Common Keston Fault Codes & What They Actually Mean
E01 / L01 – Ignition Failure or Flame Not Detected
The boiler tried to light five times and failed.
Real-world causes: frozen condensate (very common), low gas pressure on cold mornings, cracked ignition electrode, blocked burner.
E02 / L02 – False Flame or Flame Lift-Off
The boiler sees flame when there shouldn’t be one, or the flame is unstable.
Usually a sticky gas valve or cracked electrode giving false readings.
E03 / L03 – Overheat Lockout
The boiler got too hot and tripped the overheat thermostat.
Typical reasons: pump seized, air in system, blocked heat exchanger (sludge), fan not running at correct speed.
E07 / L07 – Fan Fault
The fan isn’t reaching the correct speed.
Usually a blocked venturi (spiders love them), failing fan motor, or PCB not reading the speed correctly.
E18 – Low Water Pressure
Pressure has dropped below 0.5 bar (same as F22 on Vaillant).
Fix: top up using the filling loop to 1–1.5 bar.
E37 – Condensate Trap Blocked or Siphon Full
Water can’t drain away and backs up into the boiler.
Keston twin-flue models are especially prone in winter.
E60 / E61 – PCB or Parameter Fault
The main board has detected an internal error or lost its settings.
Often needs a replacement PCB or parameter reset by an engineer.
Safe DIY Fixes You Can Do Right Now
Low pressure (E18) – find the filling loop (usually a silver hose with a small tap or key under the boiler) and top up slowly to 1.2–1.5 bar.
Frozen condensate – pour warm water over the external white pipe until it runs freely, then reset.
Reset the boiler – press and hold the reset button (spanner symbol) for 3–5 seconds.
Clear the flue terminal – safely remove leaves, snow or debris from the outside grille (ground level only).
When You Must Call a Gas Safe Engineer
Anything involving ignition, flame detection, fan, gas valve, PCB or overheat faults is strictly Gas Safe only. Keston boilers are classed as “complex appliances” because of their twin-flue design and modulating pump, so not every engineer likes working on them – always ask if they have Keston experience.
Typical 2025 Keston Repair Costs
- New ignition electrodes: £140–£220
- Gas valve: £320–£460
- Fan assembly: £280–£420
- New PCB (common on Qudos 28): £380–£580
- Condensate trap/siphon clean or replace: £120–£180
Parts are still readily available in 2025, but they’re not cheap.
How to Keep Your Keston Running Sweetly
Annual service is non-negotiable – the twin-flue system gets dust and spider webs in places other boilers don’t.
Fit a magnetic filter (even though Keston says you don’t need one – you really do).
Insulate or upgrade the condensate pipe to 32 mm if it keeps freezing.
If your boiler is over 12–15 years old and needs a PCB + gas valve, seriously consider replacement – a new Keston C40 or Heat 2 55 kW is far more efficient.
Frequently Asked Questions About Keston Faults
Why does my Keston keep going to E01 in cold weather?
99 % of the time it’s a frozen condensate pipe – the external run is often very long on flats.
Are Keston boilers still supported in 2025?
Yes – parts and technical support are still available direct from Keston (now part of the Ideal group).
Will resetting fix E03 overheat?
Only if you’ve bled the air or the pump has freed itself. If it returns instantly, call an engineer.
My Qudos 28 is 18 years old – repair or replace?
If it needs a PCB (£500+) and hasn’t been serviced regularly, replace – running costs on the old Qudos are huge compared to a new Heat 2 or Logic Max.
Where can I find a Keston-trained engineer near me?
Use the official Keston finder at keston.co.uk or call their technical helpline – they’ll point you to someone local who actually knows the twin-flue system.
Stay warm,
The Boilers2Go Team
With years of experience in boilers and home improvements in general, Myles Robinson is a passionate advocate for making home heating simpler, safer, and more affordable for households across the UK. With years of experience in the heating industry, Myles combines hands-on knowledge with a focus on customer-first service, helping homeowners choose the right boiler, understand their systems, and keep their heating running efficiently all year round. His articles break down complex topics into clear, practical advice you can trust.