What the E125 code means
On most Baxi combi and system boilers, E125 is listed as "circulation fault (primary)". The primary circuit is the sealed loop of water that carries heat from the boiler to your radiators and, on a combi, to the plate heat exchanger that makes hot water.
The boiler uses its temperature sensors to judge whether water is moving. In simple terms, if the water inside the boiler heats up much faster than it should, or the difference between the flow and return temperatures looks wrong, the boiler concludes that water is not being carried away and stops the burner. On some older Baxi and Potterton models the same code is simply labelled "pump fault", which tells you where the problem most often lies.
E125 is closely linked to E110, the overheat code. E125 is the boiler spotting poor circulation early from its sensor readings. E110 is the separate safety thermostat tripping once things have already got too hot. You may see one, the other or both.
Common causes of E125
Low system pressure
If pressure is very low, the pump cannot move water properly. This is the first thing to rule out, and you can check it yourself.
Air in the system
Air collects at high points in the system and can gather in the pump. An air lock often follows draining down, radiator changes or a big drop in pressure.
Pump sticking or failing
Pumps wear with age and can stick after sitting idle over summer. A pump that hums without turning, or runs weakly, will not keep water moving fast enough.
Sludge and blockages
Magnetite sludge builds up in older systems and restricts flow through the pump, heat exchanger, filters and valves. Radiators that are cold at the bottom are a common sign.
Closed valves
Isolating valves under the boiler left closed after work, or a system where almost every radiator is turned off, can also cut flow.
| Cause | Typical sign | Who fixes it |
|---|---|---|
| Low pressure | Gauge below 1 bar when cold | You, via the filling loop |
| Air in radiators | Cold at the top, gurgling pipes | You, by bleeding radiators |
| Radiators all turned off | E125 shortly after heating starts | You, by opening some radiators |
| Air in the pump or pump failing | Humming, knocking, no heat reaching radiators | Gas Safe engineer |
| Sludge or blockage | Cold spots at the bottom of radiators | Gas Safe engineer |
Safe checks you can do yourself
- Switch the heating off and let the boiler cool for a few minutes.
- Check the pressure gauge. If it is below 1 bar, top up to 1 to 1.5 bar using the filling loop. Our guide on how to repressurise a boiler shows how.
- Feel the radiators once the heating has run. Bleed any that are cold at the top, with the heating off, then recheck the pressure and top up if needed.
- Make sure several radiators are turned on, including the one nearest the boiler if it has a valve.
- Check the thermostat and timer are set normally.
- Press reset as your manual describes. Stop after two or three attempts.
Do not remove the casing, unscrew the pump bleed point or try to free a stuck pump. The pump sits close to live electrical parts and gas components inside the boiler, so this is a job for a Gas Safe registered engineer.
When to call a Gas Safe engineer
If the pressure is right, the radiators are bled and E125 still comes back, call an engineer. You should also call one straight away if you hear grinding or loud humming from the boiler, or if E110 starts appearing as well. The engineer will usually check that the pump is running at the correct speed, vent any air from it, check the valves under the boiler and look for signs of sludge. On an older system they may suggest a clean and a magnetic filter.
When you book, say how long the system has had the fault, whether any work was done on it recently, and what the pressure gauge reads when cold.
Why E125 often appears after heating work
If a radiator has been replaced, the system drained, or a new valve fitted, air is almost always left behind. Most of it works its way to the radiators over a few days, where you can bleed it out. Some can stay trapped in the pump, though, and that is enough to trigger E125 repeatedly. If the fault started within a week or two of work on the system, ask the installer to come back first, as venting the system properly is part of the job they did.
Signs the pump is on its way out
A pump nearing the end of its life often gives warning. Radiators take longer to warm up, the boiler becomes noisier, and E125 or E110 appears now and then before becoming a daily event. Booking an engineer at the first signs usually costs less than waiting for a complete breakdown in the middle of winter.
Typical repair costs in the UK
These are typical ranges only and vary by region and model.
| Job | Typical UK cost |
|---|---|
| Engineer call-out and diagnosis | Typically £80 to £150 |
| Venting air from the pump | Typically covered by the call-out charge |
| Pump replacement | Typically £250 to £450 fitted |
| Magnetic filter fitted | Typically £150 to £250 |
| Chemical system clean | Typically £250 to £400 |
| Powerflush | Typically £400 to £700 |
How to stop E125 coming back
Keep the cold pressure at 1 to 1.5 bar and check it monthly. Run the heating briefly once a month over the summer so the pump keeps turning. Bleed radiators each autumn and top up the pressure afterwards. Ask your engineer about a magnetic filter and inhibitor if the system is older. A yearly service is the best way to catch a tired pump before it fails. Our boiler service cost guide explains what is normally included.