Potterton E125 fault codewhat it means and how to fix it
Circulation fault (primary circuit) on every Potterton code display — the water did not warm as expected after lighting, or overshot twice. Pump, air or sludge; an engineer job after one reset.
Take care
Does the boiler run at all before E125?
The right answer depends on what you can see. Pick the one that matches.
Choose the option on the left that matches what your appliance is doing, and the answer for that case appears here.
Lights and stops within a minute, every time
The 15-second test. Pump or air.
Engineer; if the pump is silent it is the pump.
Runs for a while, then E125 after a few cycles
The overshoot test. Restricted flow — sludge or a partly closed valve.
Engineer; a filter and a clean may be proposed.
What E125 means
Potterton define this code by two tests rather than by a component, and the definition has been word-for-word the same since 2007. The first test is a flow check: the burner has been lit for fifteen seconds and the boiler temperature has not moved by a single degree, which means the heat is going nowhere. The second is an overshoot check: twice within ten minutes of lighting, the temperature has run 30° past the setting, which means the heat is arriving in a very small volume of water. Either way the manual's conclusion is the same — "poor primary circulation is indicated".
The Assure and Promax Ultra manuals stretch the first window to a range of fifteen to thirty seconds and add the code to the reset list, a one-to-three-second press; the rotary-switch manuals do not offer a reset for it at all, listing only E110, E130 and E133, so on a Promax a persistent E125 is an engineer's code from the start.
The Promax Store table adds a second line — "Tank Thermistor not in Tank" — because on a storage combi a thermistor out of its pocket produces the same no-change reading. Section B of the Promax fault-finding chart checks 230 V to the pump from the PCB, the pump supply cable, and finally replaces the pump; the Assure chart reaches "125 flashing after 1 min" and goes to section J for the same checks. The Store's manual states the conclusion the others imply: "In these instances the pump is probably inoperative."
What causes it
| Cause | What is actually happening | How likely |
|---|---|---|
| Pump not running or slow | Seized after a summer idle, or worn. The Promax chart tests for 230 V at the pump connection and replaces the pump (7684006 on the Assure) if supply is present and it does not run. | Common |
| Air in the heat exchanger | Water that is not there cannot change temperature. The chart's first instruction for E125 or E193 is "Ensure that primary circulation is OK & that there is no blockage" and that the automatic air vent cap is open. | Common |
| Heat exchanger or pipework sludged | Magnetite narrows the passages until the pump cannot push enough water through. The second trigger — twice overshooting by 30° — is the signature of a small volume of water heating too fast. | Common |
| Sensor reading a pocket of still water | On the Promax Store, a tank thermistor out of its pocket; on any model, a flow sensor coated in scale reads slowly. | Occasional |
What to try yourself
Check the pressure gauge reads between 1 and 2 bar and top up if it does not. Open every radiator valve and bleed any radiator that is cold at the top.
Assure or Promax Ultra: press Reset for about a second. Rotary knob boilers: Potterton offer no reset for E125; switching off at the spur for a minute and back on is the only owner step, and if the code returns it is an engineer's.
Put your hand on the pump body (the cylindrical motor low in the casing, if the case allows) with the boiler calling for heat. Warm and faintly humming is running; silent and cold is not. Tell the engineer either way.
Get a Gas Safe registered engineer to look at your Potterton E125
We do not pass enquiries to a repair firm, so there is no form here. Find a Gas Safe registered engineer through the Gas Safe Register, and ask to see their registration card when they arrive — it is the register, not the van, that tells you they are qualified.
What it costs to fix
Typical repair cost
£0–£600
Time on site
20 minutes
Free if bleeding and a top-up cure it. Pump £180–£300 fitted. Magnetic filter £120–£200. Power flush £350–£600.
Typical UK prices including parts and labour, updated September 2026. Your quote may differ.
Parts you may need
We may earn a commission if you buy a part through one of these links. It does not change what you pay, and it does not affect which parts we list.
Magnetic system filter, 22mm
MAG-22£65–£120Plumbing merchant
Price checked 28 September 2026
Baxi / Potterton / Main pump assembly
7684006£120–£240Authorised Baxi parts distributor
Price checked 28 September 2026
Baxi / Potterton / Main automatic air vent
7207776£15–£45Authorised Baxi parts distributor
Price checked 28 September 2026
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Related Potterton fault codes
- E110Get it checkedSafety thermostat operated — the primary water overheated — on every Potterton code display. One reset is permitted (5 s knob or 1–3 s button); a second E110 is a circulation fault for an engineer.Related: overheat and circulation
- E130Get it checkedFlue NTC operated — the flue gas got too hot — on every Potterton code display. Reset once as the user guide allows; if it returns the heat exchanger or flue needs an engineer.Related: overheat and circulation
- E193Get it checkedThe rotary-switch Pottertons' pre-ignition pump check has failed: the pump ran and the flow never proved. Same causes as E125; no owner reset is offered.Related: overheat and circulation
- E109Get it checkedThe digital-display Pottertons' pre-ignition pump check — Assure Combi, Assure System, Promax Ultra. The flow never proved, so the burner was not lit. Bleed and check pressure; then an engineer.Related: overheat and circulation
- E270Get it checkedCirculation fault (dry fire) on the Potterton Assure and Promax Ultra: the burner lit with too little water passing the heat exchanger. Not resettable by the owner; engineer.Related: overheat and circulation
- E119Get it checkedThe sealed system on a rotary-switch Potterton — Promax, Titanium, Gold, Heatmax — has dropped below half a bar. The window shows E1 and then 19. Fill to 1–1.5 bar and it clears itself.Related: same brand
Own a Potterton boiler? Save it and its codes come first next time.
Related guides
Technical review
Not reviewed by an engineer
No engineer has read this page. Severity follows what the manufacturer publishes, and we would rather tell you that than let a byline imply a sign-off.
Sources and how we verified this
We establish the facts from the sources below and then write the explanation ourselves. We do not copy manufacturer wording and we do not host their manuals.
- Manufacturer technical and service documentation
This page has not been reviewed by an engineer, and we would rather tell you that than imply otherwise. The severity rating follows what the manufacturer publishes: where they say isolate the appliance and call someone, so do we.