Keston E26 Fault Code: Causes, Fixes & Repair Costs
What does the Keston E26 fault code mean?
The E26 fault on Keston boilers — most commonly the C36 Combi — is primarily a water pressure loss warning. The boiler's onboard monitoring has detected that system pressure has been topped up more than four times within a 24-hour period, which strongly suggests an underlying leak somewhere in the heating circuit. On some C36 models or firmware versions, E26 can alternatively point to a control box (PCB) fault, so both possibilities need to be considered during diagnosis. Either way, the boiler will typically lock out and stop operating until the root cause is addressed and the unit is manually reset.
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
- Persistent system leak causing repeated pressure drops Common
The most common trigger for E26. If the boiler has needed repressurising more than four times in a day, water is escaping from somewhere in the heating system — this could be a failed seal, a corroded pipe joint, a weeping radiator valve, or a faulty pressure relief valve. Simply topping up the pressure again will not fix the underlying problem and risks water damage to flooring, plasterwork, and internal boiler components.
- Faulty pressure relief valve (PRV) Common
A pressure relief valve that is passing or stuck open will bleed water continuously, causing repeated pressure drops even without an obvious external leak. You may notice a small trickle of water from the overflow pipe outside the property. This valve requires inspection and likely replacement by a Gas Safe engineer.
- Weeping radiator or pipework joints Sometimes
Small leaks at radiator bleed valves, lockshield valves, or compression fittings in the pipework can lose water slowly enough that you only notice it through falling boiler pressure. These are not always visually obvious, especially where pipes run beneath floorboards or inside walls.
- Control box (PCB) fault Sometimes
On certain C36 models or firmware revisions, E26 is associated with a control box error rather than a pressure issue. The PCB is the electronic control unit that manages all boiler functions; if it develops a fault, the boiler will lock out and display E26. This is more likely if your system pressure appears stable and no leak can be found.
- Corroded or leaking heat exchanger Rare
Internal corrosion or scale build-up can cause the primary heat exchanger to develop small cracks or pinholes, leading to gradual water loss into the flue or combustion chamber rather than visibly onto the floor. This is a more serious and costly fault but should be considered if other leak sources have been ruled out.
How to fix it
- Check system pressure on the boiler gauge DIY safe
Look at the pressure gauge on the front of the boiler. A healthy system typically reads between 1.0 and 1.5 bar when cold. If it is below 0.5 bar, the system has lost a significant amount of water. Note how often this has been happening — if you have topped up more than three or four times recently, the E26 is likely telling you there is a leak that needs professional attention.
- Look for visible leaks around the boiler and radiators DIY safe
Check underneath the boiler, around radiator valves, at visible pipe joints, and around the pressure relief valve overflow pipe (usually a copper or plastic pipe exiting through an outside wall). Any signs of dampness, water staining, or dripping should be noted and reported to your engineer. Do not attempt to tighten fittings on gas pipework.
- Check the condensate pipe is not frozen or blocked DIY safe
In cold weather, the condensate drain pipe can freeze, which may trigger additional faults. If the external section of the pipe is frozen, you can carefully pour warm (not boiling) water over it to thaw it. This is unlikely to be the sole cause of E26 but is worth ruling out.
- Top up system pressure if it is low, then reset the boiler DIY safe
If pressure has dropped, you can repressurise the system using the filling loop (refer to your boiler handbook for the exact procedure on your C36). Bring pressure up to approximately 1.0–1.5 bar, then close the filling loop valves fully. Press and hold the reset button until the boiler attempts to restart. Only do this once or twice — if the code returns promptly, do not keep topping up. Repeated topping up masks a leak and can cause water damage.
- If a control box fault is suspected, do not attempt internal repairs Gas Safe engineer
If the system pressure appears stable and no leak can be identified, the E26 may relate to a PCB or control box fault. Diagnosis and replacement of the PCB must only be carried out by a Gas Safe registered engineer. Attempting to open the boiler casing or interfere with electronic components yourself is unsafe and will invalidate any warranty.
- Call a Gas Safe registered engineer Gas Safe engineer
If the E26 code returns after resetting, or if you have found evidence of a leak anywhere on the system, contact a Gas Safe registered heating engineer. They will pressure-test the system to locate the source of the leak, inspect the pressure relief valve, and — if no leak is found — diagnose whether a PCB fault is responsible. Do not continue topping up the pressure as a workaround.
Parts you may need
- Pressure relief valve (PRV) · from £35
- Filling loop assembly · from £20
- Radiator valve (lockshield or thermostatic) · from £15
- System inhibitor / leak sealer (minor leaks only) · from £15
- PCB / control box (Keston C36) · from £220
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–£400, depending on the underlying cause.
Frequently asked questions
Why does my Keston boiler keep losing pressure and showing E26?
The E26 code is triggered when the boiler detects that system pressure has been topped up more than four times in 24 hours — its way of telling you this is not a one-off fluctuation but a pattern suggesting an active leak. Common culprits include a weeping radiator valve, a passing pressure relief valve, or a leak at a pipe joint. If you keep topping up without finding the cause, water may be dripping onto floorboards or inside the boiler casing. An engineer can carry out a pressure test to pinpoint where the water is going.
Can I just keep resetting the Keston E26 and topping up the pressure?
Only once or twice, while you arrange professional help. Repeatedly repressurising the system without fixing the underlying leak risks hidden water damage and could eventually damage the boiler itself. The E26 code is specifically designed to flag that this pattern has become unsafe to ignore. If the code returns quickly after a reset, call a Gas Safe registered engineer rather than continuing to top up.
My Keston C36 shows E26 but I cannot find any leaks — what else could it be?
On some C36 models and firmware versions, E26 can indicate a control box (PCB) fault rather than a pressure issue. If your system pressure is holding steady, there is no visible dampness anywhere, and the PRV is not weeping, a Gas Safe engineer should check the PCB and its connections. A replacement PCB for the C36 typically costs £180–£300 for the part alone; combined with labour, total costs often fall in the £300–£500 range.
How much does it cost to fix a Keston E26 fault in the UK?
It depends on the cause. If the fix is straightforward — replacing a radiator valve or resealing a joint — most homeowners pay somewhere between £120 and £250 including parts and labour. A pressure relief valve replacement typically comes in at £150–£300. More involved repairs such as a PCB replacement can reach £300–£450 once labour is included. A leaking or corroded heat exchanger is a rare but more expensive scenario and can cost significantly more; if you are quoted for one of these, it is worth getting a second opinion and comparing the repair cost against a new boiler.