Overview
E25 on a Keston C36 combi means the boiler has detected a fault related to the control electronics (the control box / main PCB). The code is a generic control-box error and commonly indicates a faulty control board, a communication fault between the user fascia and the main control board, or a related connector/wiring problem. It can also be triggered by mains supply problems, water ingress or corrosion on connectors, or intermittent faults in sensors or other electronic components that the control box supervises. Severity is moderate to high. A persistent E25 will typically put the boiler into lockout and stop it supplying heat and hot water until the underlying fault is cleared and the boiler is reset. Because the fault concerns control electronics and the boiler’s gas and electrical systems, this is not something to leave unattended or to ‘run around’ by repeatedly trying to force operation. Basic initial checks and a reset are reasonable for a homeowner to try, but any internal electrical testing, repair, or replacement of the control box/fascia should be carried out only by a qualified Gas Safe (or local equivalent) registered engineer. Replacing or working on the control box involves isolating mains and gas supplies and requires correct commissioning and combustion checks afterwards.
Possible Cause: Control box errors – possible faulty control box
Troubleshooting Steps
Safety precautions:
1) Do not open the boiler or work on internal components unless you are a qualified gas engineer. There are live mains terminals and gas components inside that are dangerous.
2) If you smell gas, do not operate electrical switches; leave the property immediately and call your gas emergency number.
3) Use a reset or power-cycle only as described below; avoid repeatedly attempting ignition resets which can mask intermittent faults and cause repeated lockouts.
Initial homeowner checks you can safely do:
1) Note the exact error code (E25), how often it appears, and whether it appears immediately after powering up or during use. Take a photo of the display and any other codes or lights.
2) Try a single controlled reset: press the boiler reset button (follow the boiler handbook) or switch off the boiler at its electrical isolator/fuse spur for 1–2 minutes, then switch back on and observe. If the fault clears and does not return, monitor the boiler closely for a day.
3) Check the house fuse box and the boiler’s fused spur to ensure the boiler is actually receiving power and that no fuses/trip have occurred. Also check other appliances to confirm there hasn’t been a local mains problem.
4) Look for visible signs of water ingress, corrosion or burn marks around the boiler fascia, front panel and any visible cable runs. Do not remove panels to inspect internals unless you are qualified.
5) Check boiler pressure and basic system settings (thermostat, timers) to rule out unrelated causes of no-heat/no-hot-water symptoms.
If initial checks do not clear the fault (recommended next steps before calling an engineer):
1) Record fault behaviour: whether E25 appears at power-up, after a freeze/thaw, after a power cut, or intermittently. Note any other codes that appear alongside E25.
2) If you are technically qualified and competent with low-voltage work (but not gas/electrical mains), you may check that the fascia display is seated correctly and that any external fascia cable is not obviously loose. Do NOT remove covers or reconnect mains while live.
3) If the fault is intermittent, try to identify environmental triggers (humidity, condensation, recent water leaks, power surges) and address those if possible (e.g., dry out the area, ensure good ventilation).
What a qualified engineer will check and do (for your information):
1) Electrical supply checks: verify mains voltage and frequency at the boiler, check polarity, and ensure stable supply (E34/E35 related faults can cause control errors).
2) Communication and connectors: check and reseat the multi-pin connectors between the fascia/controls and the main control PCB, inspect for corrosion or damaged wires, and check the fascia PCB for faults (E41/E38 related symptoms).
3) Component and sensor checks: test flow/return/DHW thermistors and other sensors for open/short circuits, and confirm correct placement and readings during the calibration test.
4) Control box inspection: inspect the control PCB for signs of burning, blown components, or water damage. If the control box is confirmed faulty, replace it and reassemble per manufacturer procedures.
5) Post-repair checks: after any replacement, the engineer will restore power/gas, run calibration and safety tests, confirm correct ignition and combustion if applicable, and ensure no other fault codes remain.
When to call a professional and what to tell them:
Call a Gas Safe registered engineer immediately if the fault persists after a single reset, if E25 returns repeatedly, or if you see signs of water or burn damage. Provide the engineer with: boiler model (Keston C36 combi), serial number if available, the exact error code E25, when it happens, and any recent events (power cuts, freezing, leaks). Send the photo(s) you took of the display and any visible damage.
Final note: E25 is most commonly resolved by an engineer replacing or repairing the control box or fixing a communication/connector fault. Because of the gas and mains-electrical risks and the need for correct commissioning, do not attempt PCB or gas-valve replacements yourself; leave that work to a qualified engineer.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Keston C36 Combi Gas Boiler.