Overview
The E125 fault on Potterton Promax boilers means the boiler has detected a problem circulating the primary (heating) water. The controller watches how quickly the water temperature changes when the boiler fires: if the temperature fails to rise as expected or it jumps abnormally, the boiler locks with E125 to protect itself. Typical causes are low system pressure, air trapped in the heating loop, a blocked pipe or filter, a stuck or failed pump, or a faulty temperature sensor/diverter that gives incorrect readings. Severity is moderate to high for heating availability: while E125 does not usually indicate an immediate danger like a gas leak, it will stop normal heating and hot-water operation and can lead to repeated lockouts or overheating of the boiler if ignored. Some fixes are straightforward for a competent homeowner (checking pressure, bleeding radiators, a basic reset) but many causes require internal checks or replacement of components. For safety and correct diagnosis, any electrical, gas-side or internal component work should be done by a Gas Safe registered engineer; call a professional if basic checks don’t clear the fault or if you are unsure at any step.
Possible Cause: Primary water circulation fault
Troubleshooting Steps
Safety precautions
- Before doing anything, switch the boiler to the OFF or Standby position and isolate electrical supply at the fused spur if you will be opening the casing or handling internal components. Do not attempt gas-related work. If you smell gas, leave the property and call the gas emergency number immediately.
- Have basic tools, a cloth/towel, bucket and your boiler handbook to hand. If you are not confident, stop and book a Gas Safe engineer.
Initial homeowner checks
1. Note when the fault occurred and any symptoms (no heating, noisy pump, cold radiators, repeated resets). Use the reset function once: press the reset as described in your manual and observe whether the code returns.
2. Check system pressure on the boiler gauge. Most systems should sit around 1.0–1.5 bar cold; check your manual for exact target. If pressure is low, you can re-pressurise using the filling loop per the boiler handbook. Open the filling loop valves slowly and watch the pressure gauge; close the valves when pressure reaches the recommended level and secure the loop. Only do this if you are comfortable and familiar with the boiler manual.
3. Check for obvious leaks around radiators, pipes and the boiler. If pressure keeps dropping after repressurising, do not keep adding water—call an engineer.
4. Bleed radiators to remove trapped air. Start with radiators closest to the boiler and work around the system; if radiators are cold at the top this is a likely contributor.
5. After bleeding and repressurising, switch the heating demand on and listen at the boiler for pump operation: you should hear or feel the pump running (a light vibration or low hum). Also check that flow pipes from the boiler become warm. If you have zone valves, ensure they are calling for heat (thermostats on, programmers calling).
Specific diagnostic and fix steps
1. If the boiler resets temporarily but E125 returns quickly: this often points to poor circulation (airlock, partial blockage, or failing pump). Re-bleed radiators again and check the bleed valve points nearest the boiler. If the problem persists, do not repeatedly reset—call an engineer.
2. If system pressure is low and that fixed E125 until pressure dropped again: investigate for visible leaks (radiators, valves, pipework). Persistent pressure loss needs a pro to locate and repair the leak.
3. If the pump is silent when the system calls for heat or the flow pipe after the pump remains cold while the pump casing is cold: likely pump failure. You can check for pump vibration or light humming; do not strip or attempt electrical repairs. A stuck pump head can sometimes be freed by an engineer; replacement of the pump or pump cartridge is a common professional fix.
4. If the pump appears to run but flow is poor (flow pipe only lukewarm, radiators barely warm): look for blockages or magnetic dirt in the system or a blocked filter/strainer. Some systems have a magnetic filter or strainer in the return—if you know its location and are comfortable, you can isolate and drain that section following the manual, but cleaning filter housings often best left to a pro because it involves draining the system.
5. If hot-water behaviour is erratic or temperatures jump incorrectly, a temperature sensor (NTC) or the diverter valve may be at fault. These are internal components; diagnosis and replacement should be carried out by a qualified engineer.
6. If you suspect airlocks that simple radiator bleeding won’t clear, a professional power-flush or system cleanse may be needed to remove sludge and debris that block flow.
When to call a professional
- E125 persists after you have checked and corrected system pressure, bled radiators and tried a reset.
- The pump shows no sign of operation (silent) or you suspect an electrical fault with the pump or boiler controls.
- You find or suspect a leak, repeated pressure loss, or any work that requires opening the boiler casing or handling gas/electrical connections.
- You need parts replaced (pump, NTC sensor, diverter valve, PCB) or a system flush to remove sludge.
Final notes
- Do not continue to reset the boiler repeatedly; persistent lockouts indicate a fault that needs correct diagnosis. Keep records of what you tried and the conditions when the fault occurs: this helps the engineer.
- Use a Gas Safe registered engineer for any internal, electrical or gas-related repairs and for full diagnostics and safe repair of pumps, sensors and valves.
Helpful Resources
Boiler Manual
Download the official PDF manual for the Potterton Promax Combi, System, and Store.