Keston Boiler Error Codes & Faults

Keston boilers are not as widely installed as Worcester Bosch or Vaillant, but they are a fixture in thousands of UK properties, particularly new-build flats, urban apartments, and homes where a long or unusual flue run rules out a standard boiler. The twin-flue system that makes them so flexible in awkward installations is also what makes them a specialist appliance when something goes wrong.

When a Keston displays an error code, most generic boiler fault websites offer little useful help because they simply do not cover the range. This guide is for UK homeowners and landlords staring at an E-number, L-number, or a flashing red light on a Keston right now. It covers what each code means, what you can safely do yourself, and when a Gas Safe engineer with Keston experience is the only right answer.

Keston has been part of the Ideal Heating group since 2006 and production moved to Ideal’s Hull facility in 2013. Parts and technical support remain available, though they are specialist items. If you are familiar with Ideal boiler faults from the same group, the approach to fault diagnosis is similar: the ideal boiler f2 fault code explained guide is a useful reference for understanding how condensing boiler combustion faults tend to present across this family of products.

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How Keston Boilers Display Faults

Keston uses different display systems depending on the model and era of the boiler. Knowing which you have helps you interpret the fault correctly.

• Modern Keston C35 / C40 / C40P (2018 onwards): large digital display shows E** or L** codes alongside sensor readings
• Keston Qudos 28 / QSpa: small LED screen with E-codes; the Qudos 28H also has an error history function that stores the last six faults with sensor readings at the time of each lockout
• Older twin-flue Keston Celsius / Combi models: display alternates between the flow temperature and the fault number, or simply shows flashing lights with no code

All Keston boilers lock out with a solid or flashing red light when a serious fault occurs. The boiler will not attempt to restart until the reset button (marked with a spanner symbol) is pressed. On the Qudos 28H, you can access the error history by pressing the up arrow button repeatedly until the display shows ‘SHE’ (Show Errors), then pressing Enter. This is genuinely useful for tracking intermittent faults that only appear at night or during peak demand.

Keston Fault Codes: What They Actually Mean

The table below covers the most common codes across the current and recent Keston range. Note that some codes vary between models: for example, E18 indicates low pressure on most models, but the C40 and C55 use E44 for the same fault.

Code	Fault	Common Causes	DIY or Engineer?
E01 / L01	Ignition failure / flame not detected	Frozen condensate, low gas pressure, cracked electrode, blocked burner	DIY: thaw condensate, check gas. Engineer: electrode or burner
E02 / L02	False flame or flame lift-off	Sticky gas valve, cracked electrode giving false readings	Gas Safe engineer only
E03 / L03	Overheat lockout	Seized pump, air in system, blocked heat exchanger, fan fault	DIY: bleed radiators. Engineer: pump, heat exchanger
E07 / L07	Fan fault	Blocked venturi (spiders), failing fan motor, PCB speed-reading fault	Gas Safe engineer only
E18	Low water pressure	Pressure dropped below 0.5 bar	DIY: repressurise to 1.0 to 1.5 bar via filling loop
E37	Condensate trap blocked or siphon full	Blocked trap; common in winter on twin-flue models	DIY: clear condensate. Engineer if persistent
E44	Low water pressure (C40 / C55)	Pressure below safe threshold on combi models	DIY: repressurise to 1.0 to 1.5 bar
E60 / E61	PCB or parameter fault	Internal board error or lost settings	Gas Safe engineer: PCB replacement or parameter reset
E09 to E12	PCB electronic fault (C40 / C55)	Board faults detected via multi-meter diagnostics	Gas Safe engineer only

Common Keston Faults in Detail

E01 / L01: Ignition Failure or Flame Not Detected

The boiler attempted to light several times and failed. This is the most frequent fault call-out on Keston boilers, particularly during cold weather.

The most common cause in winter is a frozen condensate pipe. Keston’s twin-flue design often results in longer external condensate runs than a standard boiler, which makes freeze-ups more likely. Other causes include low gas pressure (gas meter regulators can restrict flow on very cold mornings), a cracked or worn ignition electrode, or a partially blocked burner.

What You Can Do

• Check whether other gas appliances in the property are working. If the hob is fine, the supply is likely live.
• Inspect the external condensate pipe (usually white plastic, 21 mm or 32 mm diameter). If it feels solid or has ice at the outlet, thaw it with warm water poured slowly along the pipe. Do not use boiling water.
• Once the condensate is clear, press and hold the reset button for 3 to 5 seconds.
• If the boiler lights and then locks out again within a few minutes, there is a more fundamental fault. One reset attempt is enough: call a Gas Safe engineer with Keston experience.

E02 / L02: False Flame or Flame Lift-Off

The boiler’s ionisation circuit is detecting a flame signal when the burner should not be lit, or it is reading the flame as unstable during operation. This is most often caused by a sticky gas valve that is not closing cleanly, or a cracked ignition electrode producing a false signal.

This fault is not one to keep resetting through. A sticky gas valve on a Keston’s modulating burner system can affect combustion efficiency and, in the worst case, cause an unsafe condition. A Gas Safe engineer needs to inspect the valve and electrode.

E03 / L03: Overheat Lockout

The boiler tripped its overheat thermostat because the flow temperature climbed beyond the safe limit. The most common causes are a seized or sluggish circulation pump, air trapped in the system creating hot spots, a heat exchanger blocked with sludge or limescale, or a fan that is not running at the correct speed (which affects combustion and heat transfer).

What You Can Do

• Bleed the radiators, starting from the lowest floor upwards. Release trapped air until water flows steadily, then check and top up pressure to 1.2 bar.
• Allow the boiler to cool for 15 to 20 minutes, then attempt a single reset.
• If E03 returns immediately or within a few minutes of firing, the cause is likely a pump or heat exchanger issue. Call a Gas Safe engineer.

E07 / L07: Fan Fault

The fan is not reaching or maintaining the correct speed. This matters because the fan controls air-to-gas ratio in Keston’s modulating burner: if it runs at the wrong speed, combustion becomes inefficient and the boiler locks out as a safety measure.

The most overlooked cause is a blocked venturi or air intake: spiders and insects are drawn to the warm internal air and regularly build webs that partially block the intake. This is one of the reasons annual servicing matters particularly on Keston’s twin-flue design. A failing fan motor or a PCB that cannot read the fan speed signal correctly are the other main causes.

All fan work requires a Gas Safe engineer. Do not attempt to access the fan assembly yourself.

E18: Low Water Pressure (Most Models) / E44: Low Water Pressure (C40, C55)

Pressure has dropped below 0.5 bar and the boiler has locked out to protect the pump and heat exchanger. The fix is usually straightforward.

What You Can Do

• Locate the filling loop, typically a silver braided hose with a small tap or key fitting beneath the boiler.
• Open the valve slowly and watch the pressure gauge. Close it once the reading reaches between 1.0 and 1.5 bar.
• Press the reset button to restart the boiler.
• If pressure drops again within a day or two, there is a leak in the system. Stop topping up and contact a Gas Safe engineer to locate it.

E37: Condensate Trap Blocked or Siphon Full

The condensate trap or siphon inside the boiler is full and water cannot drain away. This backs up into the boiler and triggers a lockout. Keston twin-flue models are particularly prone to this in winter because longer external runs give more surface area for condensate to pool.

The fix is to clear the trap. In mild cases, a reset after clearing an external blockage or thawing a frozen section of the pipe is enough. If the internal siphon is damaged or consistently blocking, it will need replacing. Upgrading the external condensate run to 32 mm pipe (from the standard 21 mm) makes a significant difference to how often this fault occurs.

E60 / E61 and E09 to E12: PCB and Parameter Faults

These codes point to the printed circuit board (PCB) either detecting an internal error, losing its programmed parameters, or failing to receive correct signals from sensors and components. On C40 and C55 combi models, E09 through E12 are the codes associated with PCB electronic faults.

Before assuming the PCB itself has failed, it is worth checking for a more common cause: a small internal leak that has allowed water to reach the board. Most PCB failures on Keston boilers are secondary to a dripping pump seal, a weeping heat exchanger joint, or a failed diverter valve. Fix the leak first, then assess whether the board needs replacing.

PCB replacement on a Keston requires an engineer who knows the parameter settings for that specific model. Not every Gas Safe engineer has programmed a Keston PCB, so it is worth asking before booking.

Safe DIY Fixes You Can Do Right Now

The following are the only interventions that are safe for a homeowner to attempt. Anything beyond this list requires a Gas Safe registered engineer.

• Low pressure (E18 or E44): find the filling loop beneath the boiler and top up slowly to between 1.2 and 1.5 bar. Close both valves fully before resetting.
• Frozen condensate (usually the cause of E01 in winter): pour warm water along the external pipe until it flows freely, then reset the boiler.
• Reset the boiler: press and hold the reset button (spanner symbol) for 3 to 5 seconds. Attempt a reset once only. If the code returns, stop and call an engineer.
• Clear the flue terminal: if accessible and safe from ground level, remove any leaves, debris, or snow from the external grille. Never work at height to do this.

When You Must Call a Gas Safe Engineer

Any fault involving ignition, flame detection, the fan, the gas valve, the PCB, or the overheat thermostat is Gas Safe work. These are not tasks where a second opinion or a persistent reset attempt is safe.

Keston boilers are classed as complex appliances because of their twin-flue design, modulating pump, and modulating burner. Not every Gas Safe engineer is familiar with them. Always ask whether the engineer has worked on Keston boilers before booking, and check whether they have access to Keston’s technical support line (now run through Ideal Heating).

Use the official Keston installer finder at keston.co.uk, or call the Keston technical helpline. They can point you towards an engineer in your area who knows the twin-flue system.

Typical 2026 Keston Repair Costs

Parts for current and recent Keston models remain available, but they are specialist components and priced accordingly. The table below reflects realistic all-in costs including parts and labour in 2026.

Repair	Typical 2026 Cost (inc. labour)	Notes
Ignition electrodes	£150 to £250	Common first repair on older models
Gas valve replacement	£320 to £500	Gas Safe engineer only; modulating valve on Keston is specialist work
Fan assembly	£280 to £450	Clear venturi blockage first; may resolve E07 without part replacement
PCB replacement	£380 to £600	Common on Qudos 28; check for leaks before fitting new board
Condensate trap / siphon	£120 to £200	Clean or replace; insulate external run to prevent repeat freezing
Circulation pump	£250 to £400	Grundfos integral pump on most Keston models

 

Labour rates for specialist Keston engineers tend to run slightly higher than standard boiler repair rates because of the additional diagnostic knowledge required. If a repair quote comes in at or above half the cost of a replacement boiler, it is worth at least getting a quote for a new installation before committing.

How to Keep Your Keston Running Well

Annual servicing is not optional on a Keston. The twin-flue system draws air from inside the property and has more surface area for dust, spider webs, and debris to accumulate in places other boilers never have. Skipping a year significantly increases the chance of an E07 fan fault or an E01 ignition failure the following winter.

Fit a magnetic filter if one is not already in place. Keston’s original guidance suggested their stainless steel heat exchangers did not need one, but sludge from the wider system still circulates through the boiler and accelerates wear on the pump and heat exchanger. A Magnaclean or Fernox TF1 fitted at the return is sound practice.

Insulate the condensate pipe, or upgrade it to 32 mm diameter if it keeps freezing. This is one of the most cost-effective prevention steps available for Keston boilers installed in flats, where the condensate run is often longer than on a standard house installation.

If your Keston is over 12 to 15 years old and needs both a PCB and a gas valve, the repair cost can easily approach or exceed £1,000. At that point, a new Keston C40 or Heat 2 55 kW is worth serious consideration both for efficiency and for the reliability that comes with a modern installation. When comparing the full cost picture, the moving boiler costs in your area guide covers what repositioning or replacing a boiler typically involves in terms of pipework, flue changes, and installation fees.

Frequently Asked Questions About Keston Boiler Faults

Why does my Keston keep showing E01 in cold weather?

In the vast majority of cases, it is a frozen condensate pipe. Keston’s twin-flue installation in flats often results in external condensate runs that are considerably longer than on a typical house installation, which means they freeze faster and more completely than on other boiler types. Insulating the pipe and upgrading to 32 mm diameter significantly reduces how often this happens.

Are Keston boilers still supported in 2026?

Yes. Parts and technical support are available through Keston, now operating under the Ideal Heating group. Most common components for the C35, C40, Qudos 28, and Heat 2 range are in stock, though lead times on some specialist parts can be longer than for mainstream brands. The Keston technical helpline is the most reliable route to confirming part availability for your specific model.

Will resetting fix an E03 overheat fault?

Only if the underlying cause has been resolved first. If the overheat was triggered by a temporary airlock that clears when the system cools and you bleed the radiators, then a reset may hold. If E03 returns within minutes of the boiler firing again, the pump or heat exchanger is likely the cause and a Gas Safe engineer needs to inspect it. One reset attempt is the limit.

My Qudos 28 is 18 years old. Should I repair or replace it?

If it needs a PCB replacement at around £500 and has not had regular annual services, replacement is the more sensible option. The old Qudos running costs are considerably higher than a modern condensing boiler: the efficiency gap can translate to a meaningful saving on gas bills each year. A new Keston Heat 2 or Ideal Logic Max is worth considering as a like-for-like replacement that retains the long flue-run flexibility if that is why the Keston was chosen in the first place.

Where can I find a Keston-trained engineer near me?

Use the official installer finder at keston.co.uk or call the Keston technical helpline. They maintain a list of engineers with experience on the twin-flue system and can point you to someone local. It is worth confirming that the engineer has worked on your specific model before they attend, particularly for PCB programming or gas valve work.

Does the Qudos 28H store fault history?

Yes, and this is one of the most useful features on the Qudos range. The internal memory stores the last six errors with sensor readings at the time each fault occurred and the time elapsed between them. Access it by pressing the up arrow button until the display shows ‘SHE’, then press Enter. This is particularly helpful for diagnosing faults that appear intermittently, as the stored readings give an engineer a starting point before they have seen the boiler fire up themselves.