Error E125

Safety first

1) Before you do anything, switch the boiler off and allow it to cool. Do not remove the boiler casing or attempt internal repairs. If you smell gas, leave the property and call your gas emergency number. Work only on external controls and visible pipework you are competent with.

Initial checks a homeowner can do

1) Check the system pressure gauge on the boiler. Normal cold pressure is usually around 1.0–1.5 bar (check your manual). If pressure is below 1.0 bar, the boiler may register poor circulation and lock out.

2) If pressure is low, repressurise using the external filling loop according to the boiler manual until the gauge reads about 1.0–1.5 bar. Close the filling loop and ensure the isolating valves are secured. Never over-pressurise the system.

3) Inspect visibly for leaks around the boiler, radiators, pipe joints and valves. If you find a leak, isolate the system (turn off water to the boiler if you can), turn the boiler off and call a Gas Safe engineer.

4) Bleed radiators to remove trapped air: start with the radiator nearest the boiler and work around the house. After bleeding, recheck system pressure and top up if needed.

5) Reset the boiler once you have completed the above checks (use the reset button per the manual). Then request heating/hot water and observe whether the error clears and whether radiators heat normally.

Further diagnostic checks you can safely perform

1) Listen to the boiler with a heating demand: the circulation pump usually makes a low humming sound when running. If you hear nothing at all on a call for heat, it may indicate the pump is not running. However, do not remove the casing or access live electrical parts to test.

2) If safe to do so, carefully feel the flow and return pipes (surface temperature) after a demand is made: the flow pipe should be noticeably hotter than the return if water is circulating. If both are hot and the boiler still faults, it may indicate heat transfer problems (e.g. blocked plate heat exchanger) rather than pump failure. If the flow is hot and return stays cold, there may be a blockage or diverter issue.

3) Note any unusual noises (gurgling, banging, loud knocking) or rapid temperature rises during hot water demand — these can indicate airlocks, blockages or a blocked plate heat exchanger.

What you must NOT do (safety and warranty reasons)

1) Do not remove the boiler casing, tamper with gas components, or attempt electrical checks inside the boiler — these must be done by a Gas Safe registered engineer.

2) Do not attempt to dismantle the pump, heat exchanger or diverter valve unless you are a qualified engineer.

When to call a professional and what they will check

1) Call a Gas Safe registered engineer if: repressurising and bleeding do not clear the E125; the boiler repeatedly loses pressure; you find a leak; you detect pump failure (no sound, no flow) or suspect a blocked plate heat exchanger or failing diverter, PRV or expansion vessel. Also call an engineer if the boiler trips straight back to fault when reset.

2) The engineer will safely inspect pump operation (electrical supply and rotation), check and test the plate heat exchanger for blockage and replace if necessary, examine the diverter valve and auto air vent, test the expansion vessel and pressure relief valve, and carry out powerflushing or chemical cleaning if the system is sludged. They can also check combustion and control settings after repairs.

What to record before you call an engineer

1) Note the boiler model and serial, the exact error displayed (E125), current system pressure reading, whether the fault happens on heating, hot water or both, any noises, visible leaks or when the fault first appeared. This helps the engineer diagnose faster.

Final note

If basic steps (repressurise, bleed radiators, reset) clear the fault and it does not return, continue to monitor pressure and performance. If the E125 returns or pressure keeps dropping, arrange a Gas Safe engineer promptly — circulation faults left unresolved can cause repeated overheating and more expensive damage to the boiler.