Baxi Boiler Error Code 125: Causes & Fixes

Seeing a Baxi boiler error code 125 on your digital display can be a frustrating experience, especially during a cold UK winter. This specific fault is a common occurrence across the Baxi range, including the Duo-tec, Platinum, and 600 or 800 Combi models. 

Essentially, this code is a safety signal from your appliance indicating that water is not moving through the system as it should, leading to potential overheating and an automatic shutdown.

The Baxi boiler error code 125 is formally defined as a “Primary Circulation Fault.” In the world of central heating, “primary circulation” refers to the flow of water within the boiler’s internal circuit and the heat exchanger. 

When sensors detect that the water temperature is rising too quickly or that flow has stalled, the Printed Circuit Board (PCB) triggers the 125 lockout to protect the internal components from heat damage. 

Understanding the root causes—ranging from simple pressure drops to mechanical failures—is the first step toward restoring your heating and hot water.

What does the Baxi boiler error code 125 mean?

The Baxi boiler error code 125 means that your heating system has encountered a circulation failure, preventing heated water from leaving the boiler and reaching your radiators or taps. 

Because the water isn’t moving, the heat generated by the burner has nowhere to go, causing the internal temperature to spike rapidly and triggering a safety lockout.

When the baxi boiler error code 125 appears, you will likely notice that your radiators remain stone cold and you have no hot water. The boiler’s internal sensors monitor the “gradient” (the speed at which the temperature increases); if this gradient is too steep, the system assumes the pump has failed or the circuit is blocked. 

This is a protective measure designed to prevent the heat exchanger from cracking or the safety thermostat from permanently tripping.

What are the common causes of the 125 fault?

The Baxi boiler error code 125 is typically caused by four main issues: low system pressure, trapped air (airlocks), a seized or faulty pump, or a physical blockage like limescale or sludge. Identifying which of these is affecting your appliance requires a process of elimination, starting with the most basic checks.

1. Low Water Pressure

Low system pressure is the most frequent reason for the baxi boiler error code 125. If there isn’t enough water in the system, the pump cannot create the necessary force to circulate liquid through the heat exchanger, leading to an immediate “no flow” error.

How to fix low water pressure:

1. Check the Gauge: Look at the pressure gauge on the front of your Baxi. It should ideally be between 1.0 and 1.5 bar.
2. Locate the Filling Loop: Find the silver braided flexible hose underneath the boiler.
3. Open the Valves: Slowly turn the two small lever valves (tap-like handles) until you hear water entering the system.
4. Monitor the Gauge: Watch the needle rise to roughly 1.2 bar, then close both valves tightly.
5. Reset the Boiler: Hold the reset button (usually marked with an ‘R’) for five seconds to clear the code.

2. Airlocks in the Heating Circuit

Airlocks occur when pockets of air become trapped in the pipework or radiators, acting as a physical barrier that stops water flow. This often happens after a system has been drained or if there is a minor leak that allows air to enter, eventually triggering the baxi boiler error code 125.

How to fix airlocks:

1. Turn Off the Heating: Ensure the boiler is powered down and the radiators are cool to the touch.
2. Bleed the Radiators: Use a radiator key to open the valve at the top of each radiator. Start with the ground floor and move upwards.
3. Listen for Hissing: Allow the air to escape until water begins to trickle out, then tighten the valve.
4. Re-pressurise: Bleeding radiators will drop your system pressure, so repeat the “filling loop” steps mentioned above to get back to 1.2 bar.
5. Restart: Power the boiler back on and check if the error has cleared.

3. Seized or Faulty Circulating Pump

The pump is the “heart” of your boiler, and if it fails, the baxi boiler error code 125 is inevitable. Over the summer months, pumps can “seize” due to inactivity, or the internal capacitor may fail, preventing the impeller from spinning.

How to fix a seized pump:

1. Listen for Noise: If the boiler is on but the pump is silent or making a faint humming/grinding sound, it may be stuck.
2. Access the Pump: A Gas Safe engineer will remove the boiler casing to access the pump.
3. Manual Start: The engineer may remove the large screw on the front of the pump and use a screwdriver to manually rotate the shaft to free it.
4. Check Power: If it doesn’t spin, the engineer will test the electrical supply to ensure the PCB is sending power to the pump.
5. Replacement: If the motor has burnt out, a new Grundfos or Wilo pump will need to be installed.

4. Blocked Heat Exchanger (Sludge and Limescale)

In older systems, “heating sludge” (iron oxide) or limescale can accumulate inside the primary heat exchanger. This narrows the passage for water, causing the flow to slow down and triggering the baxi boiler error code 125 due to localized overheating.

How to fix a blockage:

1. Thermal Imaging: An engineer may use a thermal camera to find “cold spots” on the heat exchanger.
2. Chemical Flush: A central heating inhibitor or cleaner can be added to the system to break down debris.
3. Power Flush: For severe blockages, a high-pressure power flush machine is used to force sludge out of the pipework and boiler.
4. Magnetic Filter: Ensure a magnetic system filter is installed to catch future debris before it enters the boiler.

Can I fix the 125 error code myself?

No, you should only attempt to fix the Baxi boiler error code 125 by re-pressurising the system or bleeding your radiators. Any task that requires removing the boiler cover or involves the internal electrical and gas components must be handled by a Gas Safe registered professional.

Attempting to repair a pump or clean a heat exchanger yourself is not only dangerous but can also void your manufacturer’s warranty. If the simple “DIY” fixes of topping up the pressure and bleeding radiators do not work, it is time to call in the experts.

Is it time to replace your old Baxi boiler?

If your appliance is frequently showing the Baxi boiler error code 125 despite multiple repairs, it might be a sign that the system is failing due to age. Constant circulation issues often point to a system heavily contaminated with sludge, which can eventually damage the PCB and gas valve.

Modern boilers are significantly more efficient than those manufactured a decade ago. Upgrading to a new A-rated model can lower your carbon footprint and reduce your monthly gas bills. If you are tired of paying for call-outs and replacement parts, a new installation with a 10-year warranty might be the most cost-effective solution.

Ready to stop worrying about repair bills? Request a new boiler quote from Boilers 2 Go today and get a fixed-price installation from local UK experts.

FAQs 

Why does my Baxi 125 error keep coming back after a reset?

If the baxi boiler error code 125 reappears shortly after a reset, it means the underlying cause—likely a failing pump or a partial blockage—has not been resolved. A reset only clears the warning; it does not fix mechanical or hydraulic issues.

Can a frozen condensate pipe cause an E125 fault?

No, a frozen condensate pipe usually triggers an E133 or E28 code. However, extreme cold can sometimes affect external pipework circulation, but E125 is almost always an internal water flow issue.

How much does it cost to replace a Baxi circulation pump?

In the UK, replacing a boiler pump typically costs between £180 and £350, including parts and labour. The price varies depending on the specific Baxi model and the brand of the replacement pump used.

Does the E125 error mean my boiler is dangerous?

The E125 code is a safety feature designed to prevent the boiler from becoming dangerous. While the error itself indicates a malfunction, the boiler has successfully shut itself down to prevent overheating or a potential fire hazard.

How can I prevent the 125 fault from happening again?

The best prevention is an annual boiler service. A Gas Safe engineer will check the pump’s health, clean the magnetic filter, and ensure the system water is treated with the correct amount of chemical inhibitor to prevent sludge buildup.