Baxi E125 Fault Code: Primary Circulation Fault
The E125 code tells you that the boiler's PCB has detected a problem with primary water circulation — essentially, it believes water is not moving through the heat exchanger at the rate it should be. When flow stalls or water temperature climbs too fast, the board triggers a safety lockout to prevent the heat exchanger and other internal components from being damaged by overheating. The boiler will stop firing completely and show E125 on the display, often alongside a flashing red light. You won't get heating or hot water until the underlying cause is resolved.
General guidance only — not a substitute for professional advice. Any gas work must be carried out by a Gas Safe registered engineer. If you smell gas or suspect carbon monoxide, leave the property and call the National Gas Emergency line on 0800 111 999.
What does the Baxi E125 fault code mean?
The E125 code tells you that the boiler's PCB has detected a problem with primary water circulation — essentially, it believes water is not moving through the heat exchanger at the rate it should be. When flow stalls or water temperature climbs too fast, the board triggers a safety lockout to prevent the heat exchanger and other internal components from being damaged by overheating. The boiler will stop firing completely and show E125 on the display, often alongside a flashing red light. You won't get heating or hot water until the underlying cause is resolved.
Common causes
- Low system pressure Common
If the system pressure drops below around 0.8 bar, the pump can't push water around the circuit effectively, causing the PCB to flag a circulation fault. Pressure loss usually points to a small leak somewhere in the system or a pressure relief valve that's weeping.
- Circulation pump failure or seizure Common
The pump is the heart of the circulation system. If it has seized — often after a period of inactivity — worn out internally, or developed an electrical fault, water simply won't move and the boiler locks out on E125. Older pumps are prone to seizing over summer when the heating hasn't run for months.
- Airlock in the system Common
Pockets of trapped air can effectively block the pump's ability to push water around the circuit. Airlocks tend to form after a system has been drained and refilled, or when pressure has been allowed to drop very low. Gurgling noises from the boiler or radiators that won't heat evenly are tell-tale signs.
- Sludge or limescale blockage Sometimes
In hard-water areas especially, limescale can build up inside the heat exchanger, and black iron oxide sludge accumulates in pipes and radiators over the years. Either can restrict flow enough to trigger E125. A system without a magnetic filter or adequate inhibitor is particularly vulnerable.
- Faulty pressure relief valve Sometimes
A pressure relief valve that opens too readily, or is stuck partially open, bleeds water from the system continuously. This causes recurring pressure loss and, by extension, recurring circulation faults. You might notice a damp patch or drip near the discharge pipe outside.
- PCB or wiring fault Rare
In a small number of cases, the fault lies not with the physical circulation at all but with the electronics. Damaged wiring to the pump, a failed pump-speed sensor, or a PCB fault can cause the board to report E125 even when the pump itself is working. This is a diagnosis of exclusion — a Gas Safe engineer will check everything else first.
How to fix it
- Check the boiler pressure gauge DIY safe
Look at the pressure gauge on the front of the boiler (or inside the casing, depending on the model). It should read between 1.0 and 1.5 bar when the system is cold. If it reads below 1.0 bar, low pressure is almost certainly contributing to the fault.
- Top up system pressure via the filling loop if pressure is low DIY safe
Locate the filling loop — usually a braided flexible hose with one or two valves beneath the boiler. Slowly open the valve(s) and watch the gauge climb. Stop when it reaches around 1.2–1.3 bar, then close the valve(s) firmly. Never overfill above 1.5 bar. If you're unsure where the filling loop is, consult your boiler's quick-start guide or look for a diagram inside the boiler casing door.
- Bleed your radiators to release trapped air DIY safe
Using a radiator bleed key, open the bleed valve on each radiator (usually at the top corner) until water starts to trickle out instead of air. Have a cloth ready to catch drips. Work from the ground floor upwards. After bleeding, re-check the boiler pressure and top up again if it has dropped below 1.0 bar.
- Reset the boiler DIY safe
Once pressure is correct and radiators are bled, press and hold the reset button for approximately 3–5 seconds (the exact method varies slightly by model — check your manual). Allow the boiler to attempt ignition. If E125 reappears within a short time, do not keep resetting — repeated resets without fixing the root cause can mask a worsening problem. Two or three attempts is the sensible limit.
- Listen for pump operation DIY safe
Shortly after a reset attempt, stand near the boiler and listen. You should be able to hear a faint hum or feel a slight vibration from the pump. Silence from the pump — combined with the boiler firing briefly then locking out — is a strong indicator of a seized or failed pump. Report this observation to your engineer as it will speed up diagnosis.
- Have a Gas Safe engineer inspect and repair the internal components Gas Safe engineer
If the fault persists after topping up pressure, bleeding radiators, and resetting, the cause is likely internal: a seized pump, blocked heat exchanger, faulty pressure relief valve, or wiring/PCB issue. All of these require the boiler casing to be removed and work carried out on gas or pressurised components. Only a Gas Safe registered engineer should do this work. Attempting it yourself is unsafe and will void your warranty.
Parts you may need
- Circulation pump (compatible replacement) · from £60
- Pressure relief valve · from £25
- Magnetic system filter (e.g. Magnaclean) · from £55
- PCB (printed circuit board) · from £180
- Central heating inhibitor (1 litre) · from £12
The exact spare depends on your boiler's GC number (on the data badge). Check this against the part before buying.
Typical repair cost
Expect to pay roughly £150–£350, depending on the underlying cause.
Frequently asked questions
Can I fix the Baxi E125 myself without calling an engineer?
There are a couple of things you can safely try yourself: topping up the system pressure via the filling loop if it has dropped below 1.0 bar, bleeding the radiators to remove trapped air, and resetting the boiler once or twice. If those steps don't clear the fault, the problem is almost certainly inside the boiler — a seized pump, blocked heat exchanger, or faulty valve — and you'll need a Gas Safe registered engineer. Opening the boiler casing and working on internal components yourself is unsafe and will void any remaining warranty.
How much does it cost to fix a Baxi E125 fault in the UK?
For most homeowners the repair bill falls somewhere between £150 and £350, which typically covers an engineer's call-out, diagnosis, and either a pump replacement or a power flush to clear a blocked heat exchanger. A straightforward fix like freeing a seized pump at the lower end of that range is quite common. If the PCB turns out to be faulty, costs can climb higher — PCBs for Baxi boilers can cost £150–£250 for the part alone plus labour — but this is a less frequent outcome. Getting two or three quotes is always worthwhile.
Why does my Baxi keep showing E125 after I reset it?
If the fault keeps returning after a reset, it means the underlying problem hasn't been fixed — you're only clearing the error code temporarily. The most likely culprits are a pump that is starting to fail (it may run briefly before stalling), a system that is losing pressure due to a leak or faulty pressure relief valve, or a significant sludge blockage restricting flow. Each reset-and-lockout cycle puts additional stress on the heat exchanger, so it's worth calling a Gas Safe engineer promptly rather than continuing to reset.
Could sludge or limescale cause the E125 code, and how is that fixed?
Yes — a build-up of black iron oxide sludge or limescale in the heat exchanger is a recognised cause of E125, particularly in older systems or those in hard-water areas. The fix is a power flush: an engineer connects specialist equipment to the heating circuit and forces a cleaning solution through at high velocity to break up and remove the debris. A power flush typically costs £300–£500 depending on system size. Fitting a magnetic inline filter and topping up with central heating inhibitor afterwards will help prevent the problem from recurring.