Keston E40 Fault Code: Causes, Fixes & Repair Costs
What does the Keston E40 fault code mean?
The E40 fault code on Keston boilers signals that the system water pressure has climbed above the acceptable upper limit. In plain terms, there is too much water in the heating circuit, or something is preventing the system from managing pressure correctly as the water heats up and expands. When E40 triggers, the boiler locks out and will not restart on its own — you must press the reset button once the underlying cause has been addressed. Ideal operating pressure on a Keston boiler should sit at around 1.0–1.5 bar when cold, typically settling near 1.3 bar.
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
- Filling loop accidentally left open Common
The filling loop is the small valve or set of valves used to top up system pressure. If it is left even slightly open after topping up, water continues to feed into the system, steadily pushing pressure beyond the safe limit and triggering E40. This is worth checking first as it is the simplest fix.
- Failed expansion vessel Common
The expansion vessel is a small sealed tank that absorbs the extra volume of water that occurs as the system heats up. If its internal diaphragm has split or the vessel has lost its nitrogen charge, it can no longer cushion that expansion. The result is a pressure spike every time the boiler fires — a classic pattern for E40. The Keston C36 installation manual specifically lists checking the expansion vessel charge and operation as the first diagnostic step for this code.
- System overfilled with water Common
If pressure is consistently high from cold — not just when the boiler is running — the system has simply been filled with too much water. This can happen after a service, a repair, or an overzealous top-up. The excess water needs to be released before the boiler will operate normally.
- Faulty pressure relief valve (PRV) Sometimes
The PRV is designed to open and vent water safely outside if pressure becomes dangerously high. If the valve has failed in the closed position it cannot do its job, allowing pressure to build unchecked. Conversely, a PRV that is weeping or has previously been manually operated may be partially compromised.
- Faulty automatic air vent Sometimes
An auto air vent that has seized or failed can contribute to abnormal pressure behaviour. While its primary job is to remove air, a failed unit can disrupt the pressure dynamics in the circuit, particularly in combination with other faults.
How to fix it
- Check the filling loop is fully closed DIY safe
Locate the filling loop — usually a braided hose or inline valve beneath the boiler. Make sure both ends or the single handle are in the fully closed position. Even a quarter-turn gap will keep admitting water. If it was open, close it and wait a few minutes for pressure to stabilise before reading the gauge.
- Read the pressure gauge and note when pressure is high DIY safe
Check the gauge when the system is cold (boiler off for at least an hour) and again when it is up to temperature. A rise of 0.2–0.3 bar during heating is completely normal. A consistent reading above 2.5–3 bar when cold, or a sharp rise of 0.5 bar or more during heating, points to different causes — overfilling versus a failed expansion vessel — and helps the engineer diagnose quickly.
- Release a small amount of water to reduce static overpressure DIY safe
If the system pressure is high even when cold (indicating overfilling), you can bleed a small amount of water from a radiator bleed valve — but release water rather than air this time. Use a cloth and a small container. Release water in short bursts, checking the gauge frequently, until pressure drops to around 1.0–1.3 bar. Do not over-bleed; releasing too much water causes the opposite problem.
- Reset the boiler DIY safe
Once you have addressed the immediate cause (closing the filling loop or releasing excess pressure), press and hold the reset button on the boiler. Do not reset the boiler more than two or three times in quick succession without resolving the root cause, as repeated lockouts can mask a worsening fault.
- Monitor pressure over the next few heating cycles DIY safe
After resetting, run the boiler through a full heating cycle and watch whether pressure climbs again. If it returns to normal and stays stable, the filling loop was the culprit. If pressure climbs again during heating and falls back when the boiler cools, the expansion vessel is the most likely cause and an engineer is needed.
- Contact a Gas Safe registered engineer to inspect the expansion vessel and PRV Gas Safe engineer
If pressure continues to rise during operation, or if your DIY steps have not resolved the lockout, a Gas Safe registered engineer needs to test the expansion vessel charge with a pressure gauge, recharge or replace the vessel if required, and inspect the PRV for correct operation. Keston boilers have a twin-flue design and modulating pump that make them more complex than standard combi boilers — when booking, ask the engineer whether they have experience with Keston appliances specifically.
Parts you may need
- Expansion vessel (internal) · from £60
- Expansion vessel diaphragm / replacement vessel (external add-on) · from £45
- Pressure relief valve (PRV) · from £25
- Automatic air vent · from £15
- Pressure gauge · from £20
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
Can I just keep resetting the Keston E40 fault to get heating back on?
Resetting will restore heating temporarily, but if the root cause is not fixed the boiler will lock out again — sometimes within minutes. Repeated resets without fixing the fault can place additional strain on components and may mask a worsening problem such as a failing expansion vessel. Limit resets to two or three while you investigate, then call an engineer if the fault returns.
My boiler pressure only rises when the heating is running — does that mean the expansion vessel has failed?
That pattern is the classic sign of an expansion vessel that can no longer absorb the thermal expansion of the water. When the vessel's diaphragm splits or it loses its pre-charge, the expanding water has nowhere to go and pressure spikes. An engineer can test the vessel charge in minutes with a simple gauge. Sometimes the vessel just needs recharging with nitrogen (cheaper, around £80–£150), but if the diaphragm has failed the whole vessel needs replacing. In cases where the fault is severe or the vessel is awkwardly positioned inside the boiler casing, costs can exceed the typical range — worth getting a quote before committing.
Are Keston boilers harder to repair than other brands?
Keston boilers are considered more complex than standard combi boilers due to their twin-flue arrangement and modulating pump. Not every heating engineer will be familiar with them. It is worth asking specifically whether the engineer has worked on Keston appliances before booking — an experienced engineer will diagnose and repair faults more efficiently and is less likely to misdiagnose the cause of an E40.
How much water pressure should a Keston boiler show on the gauge?
When the system is cold, pressure should ideally sit between 1.0 and 1.5 bar, with 1.3 bar being the sweet spot most engineers aim for. During heating you can expect a modest rise to around 1.5–2.0 bar, which is normal. If the gauge reads above 2.5 bar at any point, or above 3 bar during operation, the E40 lockout is likely and the cause needs to be investigated promptly.