Error E125

Safety precautions:

1. If you smell gas, evacuate the property immediately and call the gas emergency number. Do not attempt any work on the boiler.

2. Before opening the boiler casing or touching internal components, turn the boiler off at the mains and isolate the electrical supply. If you are not comfortable doing this, stop and call a qualified engineer.

3. Do not attempt to work on gas valves, burners, or any gas-bearing parts unless you are Gas Safe registered. Electrical or mechanical work on internal components should only be done by a competent person.

Initial homeowner checks (safe, simple steps):

1. Note the exact fault code and any other codes displayed, and the circumstances (heating on/off, hot water on, recent work). This helps the engineer later.

2. Try a simple reset per your boiler manual (usually press and hold the reset button for a few seconds). If the code clears and the boiler runs normally for a while, monitor closely; intermittent faults still need investigation if they recur.

3. Check the system pressure on the boiler manometer. For most domestic systems the target pressure is about 1.0–1.5 bar when cold (check your manual). If pressure is low, re‑pressurise using the filling loop following the boiler manual instructions, then re-run the system and see if the fault clears.

4. Check for visible leaks around pipes, radiators, valves and the boiler. If pressure drops soon after topping up, do not ignore it—find and repair the leak or call an engineer.

Bleeding radiators and removing airlocks:

1. Air in the system is a frequent cause of poor circulation. With the heating off and boiler cooled, bleed radiators starting with the radiator furthest from the boiler or the highest radiator in the house. Use a radiator key and bleed until a steady stream of water appears, then close the valve.

2. After bleeding, check and re‑set system pressure and run the heating. Repeat bleeding if necessary until the system is free of air and pressure is stable.

3. If you cannot remove air or a radiator refills with air quickly, leave it to a professional – there may be an obstruction or failing component.

Checks related to the pump (what to look/listen for):

1. With the boiler running and set to call for heat, listen at the boiler for the pump. A healthy pump usually makes a low humming vibration; a completely silent pump or loud grinding noise indicates a problem.

2. Feel the pump casing (carefully, it may be hot). A cold, silent pump when the boiler is firing suggests the pump is not powered or is seized. A hot pump that’s still not circulating may be seized or blocked.

3. Do not remove the pump or motorized parts unless you are competent. If you are mechanically experienced and only after isolating electrical supply, some users check for a stuck rotor by turning the impeller slot by hand or by powering briefly to see if it moves—but this is generally best left to an engineer to avoid damage or invalidating warranties.

Sensor and control checks:

1. If circulation and pressure look correct but the boiler still flags E125, a faulty NTC temperature sensor (flow/return sensor) can cause false circulation errors. Try resetting the boiler and note whether the fault returns under the same conditions (heating or hot water call).

2. Check for other related codes (E110 overheat, E118/E119 pressure codes) which can point to the root cause. Record these and inform the engineer.

When to call a Gas Safe registered engineer (must do):

1. If the E125 code persists after repressurising and bleeding radiators.

2. If you find a seized or noisy pump, suspect pump failure, or the pump is not running while boiler is firing.

3. If pressure repeatedly drops or you have a visible leak you cannot isolate or repair.

4. If you suspect blocked heat exchanger, diverter valve failure, faulty NTC sensor, PCB or other internal component faults.

5. If there are repeated lockouts, overheating messages, or multiple fault codes appearing together.

What the engineer will likely do and why it’s needed:

1. Verify error using service mode, inspect sensors and wiring, and run diagnostics under load.

2. Check pump operation, motor current and impeller condition; replace or rebuild pump if seized or failed.

3. Flush or power‑flush the system if sludge or scale is restricting flow; clean or replace magnetic filter if fitted.

4. Replace faulty NTC sensors, diverter valves, or other components as required. Test the heat exchanger and check for blockages.

5. Test system pressure, expansion vessel and safety devices to ensure stable operation.

Final notes and immediate actions:

1. If a simple reset + repressurise + radiator bleed fixes the fault and the boiler runs normally for several cycles, continue to monitor pressure and performance. If the fault returns within days, book an engineer.

2. Keep a record (dates, times, actions taken, photos) to help the engineer diagnose intermittent faults faster.

3. Never work on gas or sealed parts of the boiler unless you are qualified. Persistent E125 faults commonly require a Gas Safe engineer to diagnose and repair pump, valve, sensor or internal blockages.