Overview
The E125 fault code on Potterton Promax Ultra / Gold / Titanium combi and system boilers indicates a primary water circulation fault. The boiler has detected abnormal behaviour in the primary heating circuit — either the water temperature is not rising fast enough after the burner starts (less than a 1°C change in 15 seconds) or the flow temperature has jumped very rapidly (a 30°C overshoot twice in a 10-minute period). That detection is a safety/protection response to poor circulation rather than a direct gas or ignition fault. Severity ranges from mild to serious. In many cases the cause is straightforward and can be corrected by the homeowner (low system pressure, trapped air in radiators) so the boiler will return to normal after repressurising or bleeding. However the code can also be caused by a stuck or failed pump, blocked pipes/heat exchanger, faulty NTC temperature sensor or diverter valve problems. Those faults can cause repeated lockouts, overheating trips or component damage if the boiler is repeatedly reset and run while circulation is inadequate, so caution is required. Simple checks and a single reset are reasonable DIY actions; any suspected pump, sensor, heat-exchanger or gas-related repair should be left to a Gas Safe registered engineer.
Possible Cause: Primary water circulation fault
Troubleshooting Steps
Safety precautions:
- If you smell gas, leave the property, call the gas emergency number and a Gas Safe engineer — do not attempt to operate the boiler.
- Do not open the boiler casing or attempt any work on gas components, wiring or the pump internals unless you are qualified. Electrical isolation may be necessary for some checks; if you are not confident, stop and call a professional.
- Limit resets: press the reset once and observe. Repeated resets without diagnosing the cause can damage components.
Initial homeowner checks you can do safely:
1) Note the error conditions and symptoms: is there no heating, no hot water, are radiators cold at the top, any unusual noises from the boiler or pump? Record the boiler pressure reading from the gauge.
2) Check system pressure: the cold operating pressure should normally be around 1.0–1.5 bar (consult your manual for the exact range). If pressure is below 1.0 bar, the boiler may flag a circulation error. If low, repressurise using the filling loop according to the boiler manual until pressure reaches the recommended level, then close the loop and re-check.
3) Bleed radiators: trapped air can cause circulation faults. Starting at the highest radiators, use a radiator key to bleed until water (not air) appears, then re-check and top up system pressure if required.
4) Reset the boiler: after completing pressure and bleeding checks, use the boiler reset as per the manual. Observe whether the E125 returns and whether the boiler runs normally for a period.
Specific diagnostic checks and next steps:
1) Listen to the pump: when the boiler attempts to fire, you should hear/feel the pump running. A totally silent pump when the boiler calls for heat suggests pump failure or no power to the pump. A vibrating or noisy pump can indicate seizure or fault.
2) Check for visible leaks or blocked pipework: leaks will lower pressure and affect circulation; visible blockages or kinks in exposed pipework can impede flow.
3) Check flow/return temperatures if accessible: a very large temperature difference or a flow temperature that stays low while burner fires supports a circulation issue.
4) Check error trigger behaviour: if the boiler reports rapid temperature jumps (30°C overshoots) or a failure to change temperature by 1°C in 15 seconds, that points to either a sensor (NTC) measuring incorrectly or real lack of flow. If your pressure and bleeding checks are good but the code returns quickly, the pump or NTC sensor may be suspect.
When to call a professional (Gas Safe engineer):
- If pressure/bleeding/reset did not clear the fault or the fault returns frequently.
- If the pump is silent, noisy, or appears seized — pump replacement/repair requires a trained engineer.
- If you suspect a blocked plate heat exchanger, diverter valve fault, or failed NTC temperature sensor. These require specialist diagnosis and component replacement.
- If the boiler repeatedly locks out or shows other error codes alongside E125.
What to tell the engineer and what they will do:
- Provide the error code (E125), what you tried (pressure reading, repressurised, bled radiators, reset), symptoms (no heat, cold radiators, unusual noises), boiler model and age.
- A Gas Safe engineer will safely isolate and open the casing, check pump operation and wiring, check NTC sensors, test flow and return temperatures under load, inspect the diverter valve and plate heat exchanger for blockages, and run controlled tests. They may clean/fluid-flush the system, replace the pump head or whole pump, replace sensors or repair/replace blocked components as appropriate.
Final note: E125 is a circulation protection code — quick homeowner checks (pressure, bleeding, one reset) are appropriate, but do not continue to run or repeatedly reset the boiler if the fault returns. For pump, sensor, valve or internal blockages call a Gas Safe registered engineer to diagnose and repair safely.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Potterton Promax Ultra, Gold/Titanium Combi & System.