Baxi Boiler 125 Fault Code


The Baxi fault code 125 water circulation Q & A or require an engineer for an emergency boiler repair.


The Baxi Boiler Fault Code 125 Error Common Q & A


Just below, we have a list of common questions relating to the 125 fault code on the Baxi boiler.

Baxi Boiler 125 Fault Code

  • What does the 125 fault code mean on my Baxi boiler
  • What causes a water circulation fault?
  • Can the Baxi 125 fault code be fixed on a DIY basis?
  • Will resetting the boiler fix the 125 fault code on my Baxi boiler?
  • How will I know if my boiler has lost, or is losing boiler pressure?
  • What should I do if my boiler pressure is below 1.3 bar?
  • If my boiler is losing pressure, what’s the probable cause?
  • How can I find and fix the leak that’s causing my boiler to lose pressure?
  • There are drops of water beneath my boiler, what is the fault likely to be?
  • Why do boiler pumps leak, and can they be fixed?
  • Why would the heat exchanger crack and leak?
  • Can a cracked heat exchanger be repaired?
  • How can I check for airlocks in radiators?
  • How will a Gas Safe engineer check to see if there’s a fault with the pump?
  • What needs to be done to determine which fault is causing the Baxi 125 fault code?
  • Are electrical faults with heating pumps hard to diagnose?

What does the 125 fault code mean on my Baxi boiler?


According to Baxi, the 125 boiler fault code occurs when there is an issue with water circulation.

Once the boiler heats water, it’s pumped around the heating system (pipework, towel rails and radiators).

When the fault code is displayed on the control panel, that’s a sign of bad/no circulation.


What causes a water circulation fault?


The most common causes of a circulation fault are:

  • A loss of, or low boiler pressure
  • Air locks in the heating system
  • Heating pump faults

Can the Baxi 125 fault code be fixed on a DIY basis?


There are several ways to try and clear the 125 fault code.

However, anything that involves removing the boiler’s casing is classed as working on a gas appliance.

When this is the case, a Gas Safe registered engineer will need to be called in to diagnose and fix the fault.


Will resetting the boiler fix the 125 fault code on my Baxi boiler?


The reset function on boilers is only designed to be used when a fault has been found and fixed.

Resetting the boiler without finding the root cause won’t fix the boiler, or clear the 125 fault code.


How will I know if my boiler has lost, or is losing boiler pressure?


Most modern boilers will need to be set to 1.3-1.6 bar to operate correctly.

Locate the boiler pressure gauge (usually on the front of the boiler), and check the pressure.

If the boiler pressure gauge is displaying less than 1.3 bar, there’s a good chance it’s lost pressure, or hasn’t been set to the correct pressure to begin with.

A loss in boiler pressure is usually obvious. You’ll notice the gauge dropping consistently over days/weeks.


What should I do if my boiler pressure is below 1.3 bar?


To determine if you have a boiler that’s losing pressure, the pressure needs to be set correctly, and then monitored.

Underneath your boiler you’ll see the external filling loop. This is a braided hose with a valve on it. Opening this valve will add water to the system, and increase the pressure.

Once the boiler has been set to 1.3-1.6 bar (always check your owner’s handbook to determine the correct pressure), you’ll need to monitor the pressure over the space of a few days, to determine if your boiler is losing pressure.


If my boiler is losing pressure, what’s the probable cause?


When a boiler loses pressure the most probable cause is a leak.

Leaks can come from any part of the central heating system including:

  • Auto air vents
  • Pressure relief valves
  • Expansion vessels
  • Weak soldered joints
  • Pin holes in radiators or towel rails
  • Radiator or towel rail valves
  • Blown heating pump seals
  • A cracked heat exchanger

How can I find and fix the leak that’s causing my boiler to lose pressure?


A visual inspection of all pipework, radiators, towel rails and valves will help to highlight any obvious leaks.

If you find a leak when inspecting the heating system, the system needs to be drained, the culprit made water tight, and the system refilled.

Sometimes the fault could be something as simple as an olive that’s become displaced on a radiator valve, or new valve that’s not had enough PTFE tape added during installation.


There are drops of water beneath my boiler, what is the fault likely to be?


If you’ve got a boiler that’s dripping water, there’s a good chance that the issue lies with either the pump, or a cracked heat exchanger.

Any loss in water from the system, will reduce the boiler’s pressure.


Why do boiler pumps leak, and can they be fixed?


Older pumps are known to blow their seals. Once the seals are blown, water will start to drip from the pump onto boiler parts.

Although fixing pump seals isn’t a hugely expensive job, it’s something that should be done as soon as possible. If a pump is leaking, it could be depositing water on expensive electrical components.

A typical example would be the boiler’s PCB. If this gets water damaged it could cost upwards of £400 to fix.

If the pump is particularly old a replacement might be in order, rather than just replacing the seals.


Why would the heat exchanger crack and leak?


Unless old-age can be blamed, cracked heat exchangers usually stem from limescale build-up.

This build-up comes from minerals in water. They can settle on parts such as the heat exchanger and when they do, they create hot spots.

As the heat exchanger heats up and cools down, these hot spots start to create weak spots, until the heat exchanger eventually cracks. And it’s via these cracks that the heat exchanger starts to leak.


Can a cracked heat exchanger be repaired?


Unfortunately, it’s unlikely your heating engineer is going to repair a broken heat exchanger. Instead, they’ll replace it.

A replacement heat exchanger can cost upwards of £450.


How can I check for airlocks in radiators?


Sometimes the fix for low boiler pressure is an easy one; like bleeding radiators.

A radiator with an airlock in it will either:

  • Not get up to temperature
  • Get hot at the bottom, but stay cold at the top

This isn’t to be confused with central heating sludge build-up. When this is the cause, you’ll have the same issue with radiators not getting up to temperature.

But, you’ll find radiators getting hot at the top, and cold at the bottom (i.e. the opposite way around when compared with air locks).

That’s because air locks occur at the top of radiators (not allowing the top to heat up), whereas heating sludge settles at the bottom (not allowing the bottom to heat up).

A simple check is to use a radiator bleed key to slowly open one of the bleed valves on a radiator. If it hisses, that’s a sign of air. If it instantly dribbles water, that’s a sign that there’s no air (in that particular radiator).

If you suspect air locks are a problem, you’ll need to bleed every towel rail and radiator in your property.


How will a Gas Safe engineer check to see if there’s a fault with the pump?


As we mentioned earlier, leaking pumps are common. So, that’s the first thing that needs to be checked, and fixed if there’s a leak present.

Like radiators and towel rails, pumps can get air locks too. If you’ve noticed the pump knocking or banging recently, that’s a good indication of an air lock.

Luckily, bleeding air from the pump isn’t a long, difficult or expensive process. They have a bleed screw that allows for quick release of air.

If the Baxi boiler 125 fault code is still present after bleeding the pump, there could be a fault stopping the pump from working correctly such as:

  • Partially or fully seized motor
  • Incorrect installation causing a partial seizure on the pump’s shaft
  • Lack of communication between the pump and the PCB (i.e. an electrical fault)

What needs to be done to determine which fault is causing the Baxi 125 fault code?


A seized motor or shaft can be checked by turning the pump on the shaft. If the cycle isn’t smooth, there’s a good chance either the shaft’s bearings need replacing, or the pump’s motor is seized and needs cleaning out or replacing.

These issues can also arise due to incorrect installation. The pump’s shaft needs to be horizontal. If the pump was installed even a few degrees off horizontal, two things will happen:

  • The pump won’t function correctly
  • The motor, shaft and shaft bearings will have been put under excess strain and will wear out prematurely

If the pump is struggling to operate due to incorrect installation, it can be reinstalled.

However, it might be that the pump’s shaft, bearings and motor have been put under too much strain and the whole unit needs to be replaced.


Are electrical faults with heating pumps hard to diagnose?


The first step here is to visually inspect wiring and connections too and from the pump. Frayed wires will need to be replaced and any loose connections need to be re-secured.

If there’s no obvious damage, your Gas Safe engineer can test the circuit to see if there are any faults, using a multi meter.

The Baxi 125 fault code relates to a circulation fault, but that doesn’t mean the pump is always to blame.

It could just as easily be a fault with the PCB, miscommunicating with the pump and not telling it to fire into life when it should.

Other Baxi boiler issues


For more boiler issues regarding the Baxi Boiler then please visit the Baxi Boiler Problems And Cures page.