Overview
E25 on a Keston C40/C55 combi means the boiler has detected a rapid rise in the flow temperature that exceeds the safe limits and has gone into a fault/lockout to protect itself. The boiler monitors flow and return temperatures and if the flow temperature climbs too quickly the control electronics will interrupt operation and show E25 so damage or overheating is avoided. The most common reasons for this fault are circulation problems in the central heating circuit: an airlock, blocked or closed valves, a failed or seized pump, or restriction in the heat exchanger or system (including magnetic filters or debris). Less commonly it can be caused by a faulty sensor/thermistor or an internal control box fault. Severity is moderate to high — the boiler has protected itself, but persistent circulation failures can cause overheating, additional damage or repeated lockouts. Some basic checks (reset, pressure, bleeding radiators, checking external pump valves) are safe for a competent homeowner, but diagnosing or repairing internal components (pump, control box, thermistors, gas or electrical internals) should be done by a Gas Safe engineer.
Possible Cause: Flow temperature increased too quickly – check water circulation
Troubleshooting Steps
Safety precautions:
1) Do not remove the boiler casing or work on gas or live electrical components. Only a Gas Safe registered engineer should open the boiler or replace internal gas or electrical parts.
2) Before doing any hands-on checks allow the boiler and system to cool to avoid scalding. Turn the boiler off at the programmer or the main on/off switch for non-invasive checks. If you need to isolate mains power for any work, switch off the consumer unit circuit but do not attempt internal repairs.
3) If you smell gas or suspect a leak, leave the property, call the emergency gas number and Gas Safe immediately.
Initial homeowner checks you can do safely:
1) Note the exact fault code and when it happened. Press the boiler Reset once (follow the manual) and see if the boiler restarts and runs normally. Do not repeatedly reset – one attempt is enough.
2) Check system water pressure on the boiler gauge. Typical target is about 1–1.5 bar when cold (check your manual for the exact range). If pressure is very low (under about 0.8 bar) top up following the manual and recheck for leaks. If pressure repeatedly falls, call an engineer.
3) Look and listen: while the boiler is trying to run, stand near the boiler (without touching it) and listen for the pump running. If you can access any external system pump isolating valves, confirm they are open.
4) Bleed a few radiators to remove trapped air. Airlocks are a common cause of poor circulation and a rapid flow temperature rise. When bleeding radiators, turn the boiler off and allow cooling; use a cloth and a bleed key and re-pressurise if pressure drops.
5) Ensure all radiator thermostatic valves and lockshield valves are open enough to allow water flow. If the central heating zone valves or thermostats are closed, open them.
6) Check any visible magnetic filter/strainer on your system (not inside the boiler). If fitted and accessible, check for blockages per the filter manufacturer’s instructions (do not remove internal boiler strainers).
Specific diagnostic and next steps:
1) If the boiler restarts and runs but E25 returns soon after, this strongly indicates poor circulation rather than a one-off glitch. Record whether the fault happens on heating, hot water, or both, and whether it occurs immediately or after running a short time.
2) If flow temperature on the boiler display rises very quickly while return stays low (large flow/return delta), suspect pump failure, airlock or blockage. A blocked heat exchanger or closed/partially closed diverter/zone valve can cause this.
3) If you can safely check the external pump (not the internal boiler pump) ensure its isolating valves are open and the pump is powered. You may hear or feel a small vibration if it is working. Do not try to spin the pump by hand if it is live.
4) After bleeding radiators and checking valves, re-pressurise the system and try running the boiler. If E25 is cleared and no further symptoms appear, monitor the system for recurrence.
5) If the fault persists after these checks, or if you suspect the pump inside the boiler, blocked heat exchanger, or electrical/control box/thermistor fault, stop attempting DIY repairs. The Keston manual and fault charts also list E25 alongside possible control box errors and thermistor faults, which require an engineer.
When to call a professional (and what to tell them):
1) Call a Gas Safe registered heating engineer if E25 will not clear after the simple system checks above, if the pump appears dead, if you cannot remove an airlock by bleeding, or if the fault returns repeatedly. Do not attempt internal component replacement yourself.
2) Tell the engineer the exact fault code (E25), what you observed (rapid flow temperature rise, whether the pump was audible, system pressure, whether you bled radiators, and any recent work on the system). That information speeds diagnosis.
3) The engineer will safely isolate and open the boiler, test thermistors/flow sensor, confirm pump operation or replace the pump, check heat exchanger flow for blockages, check diverter/zone valves, and if needed replace or repair control electronics. They will also run combustion and electrical checks as required.
Important notes:
1) Do not repeatedly reset the boiler to keep it running; that can hide an underlying fault and may cause further damage. One reset is acceptable after a cooling period and initial checks.
2) Because E25 can be caused by simple circulation issues (air, closed valves, low pressure) it is worth doing the basic checks above first, but if the fault is not resolved by those steps, a Gas Safe qualified engineer must carry out further diagnosis and any repairs involving internal components, gas, or mains electricity.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Keston C40 C55 Combi Gas Boiler.