Keston E04 Fault Code: Causes, Fixes & Repair Costs
What does the Keston E04 fault code mean?
The E04 code on a Keston boiler is a status notification rather than a fault with a specific component. It appears when the mains power to the boiler has been switched off and back on while the boiler was already sitting in a lockout state from a pre-existing fault. In other words, E04 is telling you that someone cycled the power instead of pressing the dedicated reset button to clear the original lockout. The code itself does not tell you what caused the initial lockout — that underlying fault (such as low system pressure, an ignition failure, or an overheat condition) still needs to be identified and resolved before the boiler will run normally again.
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
- Mains power cycled instead of using the reset button Common
The most common route to seeing E04 is a simple mistake: when a boiler goes into lockout, many homeowners instinctively flick the mains switch off and on again. On Keston boilers this does not clear a lockout — it generates the E04 notification instead. The correct approach is always to press the dedicated reset button on the control panel.
- Underlying low system pressure caused the original lockout Common
A drop in system pressure below around 0.5 bar is one of the most frequent reasons a Keston boiler locks out in the first place. If the pressure fault was not resolved before power was cycled, E04 will appear on top of the original issue. Keston boilers operate best with cold system pressure between 1.0 and 1.5 bar, visible on the pressure gauge on the front of the unit.
- Ignition or flame-detection failure triggered the initial lockout Sometimes
If the boiler failed to light or lost its flame signal during a heating cycle, it would have locked out with its own fault code. If power was then switched off and on without pressing reset, E04 overlays that original code. The ignition or flame detection issue still needs a Gas Safe engineer to investigate.
- Overheat or sensor fault caused the original lockout Sometimes
Overheat protection, faulty thermistors, or a blocked condensate pipe can all trigger a lockout. Cycling the mains rather than pressing reset after such an event produces E04. If the root cause was an overheat condition, simply resetting the boiler repeatedly without investigation is not safe and an engineer should be called.
How to fix it
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler. The needle should sit between 1.0 and 1.5 bar when the heating is cold. If it reads below 1.0 bar, use the filling loop (usually a braided flexi-hose beneath the boiler) to slowly top up the pressure until the gauge reads around 1.2 bar, then close the filling loop valves. Never overfill beyond 2.0 bar.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in your home — such as a gas hob — are working normally. If you have no gas supply at all, contact your gas supplier rather than attempting any work on the boiler.
- Check for a frozen or blocked condensate pipe in cold weather DIY safe
If the temperature has been below freezing, inspect the plastic condensate pipe that runs outside or through an unheated space. A frozen pipe is a very common cause of lockouts in winter. You can carefully pour warm (not boiling) water over the outside section to thaw it, or wrap it in a warm damp cloth. Once thawed, proceed to the reset step.
- Reset the boiler correctly using the reset button — not the mains switch DIY safe
Locate the reset button on the control panel. On most Keston models this is a clearly labelled button; press and hold it for around 3 to 5 seconds until you hear a click or see the display change as the boiler begins its restart sequence. On models where the reset function is built into the main control dial, rotate the dial to the reset symbol, hold it briefly, then release. Do not cycle the mains power again — this is what generated the E04 in the first place.
- Observe the boiler through its startup cycle DIY safe
Watch and listen as the boiler attempts to restart. If it fires up and runs normally, monitor it over the next hour or so to confirm it stays running. If the same fault code or a different error appears within a short time, note the code displayed as this will help an engineer diagnose the underlying problem.
- If the fault returns or a new code appears, call a Gas Safe registered engineer Gas Safe engineer
A recurring lockout means the root cause has not been resolved. Do not reset the boiler more than two or three times in total. A Gas Safe registered engineer will need to read the boiler's fault history, test components such as the ignition system, gas valve, sensors, fan, and PCB, and carry out any necessary repairs. Keston boilers have a twin-flue design and a modulating pump that not all engineers are familiar with — when booking, ask whether the engineer has experience with Keston appliances.
Parts you may need
- Keston pressure relief valve · from £35
- Keston thermistor / temperature sensor · from £45
- Keston ignition electrode assembly · from £55
- Keston PCB (control board) · from £280
- Keston gas valve · from £160
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 £80–£350, depending on the underlying cause.
Frequently asked questions
Is E04 actually a fault, or just a notification on Keston boilers?
E04 is best described as a status notification rather than a fault with a physical component. It tells you that the mains power was switched off and on while the boiler was already in lockout — but it does not identify the original cause of that lockout. Once you reset the boiler correctly and it runs without issue, E04 has done its job. If a different fault code appears after reset, that code is the one to investigate.
Why does my Keston boiler show E04 every time I turn the power back on?
Because switching the mains off and on does not clear a Keston lockout — it simply generates the E04 notification each time you do it. You need to press the reset button on the control panel (or turn the dial to the reset symbol on applicable models) to properly clear the lockout. If the boiler then locks out again quickly, there is an underlying fault that needs a Gas Safe engineer to diagnose.
Could low boiler pressure be why I'm seeing E04?
Yes, this is one of the most common real-world paths to E04. The sequence usually goes: pressure drops below the safe threshold, the boiler locks out, someone switches the power off and on to try to fix it, and E04 appears. Check your pressure gauge — it should read between 1.0 and 1.5 bar when the system is cold. If it is low, top up via the filling loop, then use the reset button (not the mains switch) to restart the boiler.
How much will it cost to fix the fault behind an E04 on a Keston boiler?
Because E04 is a secondary notification, the repair cost depends entirely on what the original lockout fault was. A straightforward repressurise and reset costs nothing or at most a standard call-out fee of around £60–£100. Most common repairs — replacing a sensor, ignition electrode, or similar part — tend to fall in the £80–£350 range including labour. If the underlying fault turns out to involve a PCB or gas valve, costs can be higher; on older Keston C-series boilers it may be worth comparing repair costs against replacement. Always use a Gas Safe registered engineer for any component work.