Biasi E04 Fault Code: Causes, Fixes & Repair Costs
What does the Biasi E04 fault code mean?
The Biasi E04 fault (sometimes shown on the display as ER 04) signals that the boiler's primary circuit has encountered a problem with water circulation or system pressure. Specifically, the boiler's PCB has detected either insufficient water pressure, a failure in the pump's ability to circulate water around the system, or — less commonly — that the water pressure sensor is not connected or reading correctly. The result is a full boiler lockout: the boiler shuts itself down as a safety measure and will not fire until the underlying issue is resolved. This code is one of the more common faults on Biasi boilers and covers a range of causes from something as simple as a pressure drop to a worn pump or a cracked heat exchanger.
General guidance only — not a substitute for professional advice, and not certified by a Gas Safe engineer. Always have your specific appliance assessed by a qualified professional. 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.
Common causes
- Low system pressure Common
Biasi boilers should operate with a cold system pressure of around 1 to 1.5 bar — most engineers set them at approximately 1.3 bar. The boiler will typically continue to function when pressure dips slightly below 1 bar, but once it drops below around 0.5 bar the PCB triggers a lockout to protect the heat exchanger and pump. Pressure naturally drops very slowly over time, but a sudden or repeated drop usually points to a leak somewhere in the system.
- System leak causing pressure loss Common
A slow leak anywhere in the central heating pipework, radiator valves, or within the boiler casing itself can steadily reduce system pressure until the E04 lockout is triggered. Internal boiler leaks — for example from pump seals, the pressure relief valve, or the heat exchanger — are particularly common on older Biasi units and can be difficult to spot without removing the boiler casing.
- Faulty or seized pump Common
The pump circulates hot water from the boiler around the radiators and back again. If it has seized, is blocked with heating sludge, or has developed an airlock, water flow will be inadequate and the pressure sensor will flag the fault. Pumps on Biasi boilers that have been run at persistently low pressure are at higher risk of premature wear because they have to work harder to maintain flow.
- Blocked system or airlock Sometimes
A build-up of magnetite sludge or limescale in the system pipework, radiators, or heat exchanger can restrict water flow enough to mimic a pump fault. An airlock trapped in the pump or pipework will have the same effect. Systems without a magnetic filter or inhibitor are especially prone to sludge build-up over time.
- Faulty pressure gauge or pressure switch Sometimes
If the pressure gauge or pressure switch itself is giving inaccurate readings — either due to wear, corrosion, or a loose connection — the boiler may lock out on E04 even when the actual system pressure is within the normal range. This is less common but worth considering when pressure appears correct yet the fault persists.
- Cracked or leaking heat exchanger Rare
A heat exchanger that has developed hairline cracks due to wear, limescale stress, or overheating can cause a slow internal leak. This gradually reduces system pressure and may also allow water to reach other electrical components inside the boiler. Heat exchanger failure is one of the more serious and costly causes of a recurring E04 fault.
How to fix it
- Check the pressure gauge on the boiler DIY safe
Look at the pressure gauge on the front of the boiler. It should read between 1 and 1.5 bar when the system is cold. If it reads below 1 bar — and especially if it is at or near zero — low pressure is almost certainly triggering the E04 lockout.
- Repressurise the system using the filling loop DIY safe
If the pressure is low, locate the filling loop (usually a silver or grey braided hose with one or two small valves, found beneath the boiler). Slowly open the valve(s) and watch the pressure gauge rise. Stop once it reaches around 1.2 to 1.3 bar. Close the valve(s) firmly. If you are unsure how to do this, refer to your Biasi boiler's user manual for model-specific instructions. Do not overfill — pressure above 2.5 bar can open the pressure relief valve.
- Reset the boiler DIY safe
Once pressure is at the correct level, attempt a reset. Most Biasi models have a reset button on the front panel — press and hold it for a few seconds as described in your manual, or switch the boiler off at the mains, wait about 10 seconds, then switch it back on. If the boiler fires up and runs normally, monitor the pressure over the next few days. If E04 returns or the pressure drops again quickly, there is an underlying fault that needs professional attention.
- Check that the gas supply is working DIY safe
While E04 is primarily a pressure and circulation fault rather than a gas fault, it is worth confirming that other gas appliances in your home (hob, gas fire) are working normally, to rule out a broader gas supply issue before calling an engineer.
- Thaw a frozen condensate pipe if relevant DIY safe
This step only applies in cold weather. The condensate pipe runs from the boiler to an external drain and can freeze solid in winter, causing lockout. If the boiler is also displaying symptoms of a condensate blockage — a gurgling noise before lockout — you can carefully pour warm (not boiling) water along the external section of the pipe to thaw it. However, this is not the primary cause of E04.
- Call a Gas Safe registered engineer Gas Safe engineer
If repressurising and resetting does not resolve the fault, or if the pressure keeps dropping within a few days, you need a Gas Safe registered engineer. They will carry out a full system pressure test to locate any leaks, inspect and test the pump and pressure switch, check for sludge or airlock issues, and examine the heat exchanger. They can also add central heating inhibitor and fit magnetic and limescale filters to protect the system going forward. Do not continue resetting the boiler repeatedly — this will not fix the underlying fault and can mask a worsening problem.
Parts you may need
- Central heating pump (e.g. Grundfos UPS2 or equivalent) · from £65
- Pump head seal kit · from £15
- Water pressure switch / sensor · from £25
- Magnetic system filter (e.g. Adey MagnaClean Pro2) · from £75
- Central heating inhibitor (e.g. Fernox F1, 1 litre) · from £12
- Filling loop assembly · from £18
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 £100–£320, depending on the underlying cause.
Frequently asked questions
Can I fix a Biasi E04 fault myself?
The only steps a homeowner can safely carry out are checking and topping up the system pressure via the filling loop, and resetting the boiler. If either of those actions resolves the fault and it does not return, you may not need an engineer. However, if pressure drops again within a few days, or if the fault returns after resetting, you have an underlying problem — a leak, a pump fault, or a blocked system — that requires a Gas Safe registered engineer to diagnose and repair.
Why does my Biasi boiler keep losing pressure and showing E04?
Recurring pressure loss almost always means there is a leak somewhere. This could be a small weep from a radiator valve, a joint in the pipework, the boiler's own pressure relief valve (which can drip if it has been opened too many times), or an internal leak within the boiler such as a worn pump seal or cracked heat exchanger. Topping up the pressure will clear the immediate fault, but the leak must be found and fixed to stop it coming back. An engineer can perform a pressure drop test to pinpoint the source.
How much does it cost to fix a Biasi E04 fault?
Most people with an E04 fault pay somewhere between £100 and £320 for the repair, depending on what is causing it. A straightforward call-out to repressurise, add inhibitor, and fit a magnetic filter typically falls at the lower end. Replacing a faulty pump — including labour and a quality replacement such as a Grundfos unit — usually comes to around £200 to £280. A pressure switch or sensor replacement is generally less. If the heat exchanger is cracked and needs replacing, costs can exceed £400 to £500, and at that level it is often worth getting a new boiler quote as well, particularly on older Biasi models.
Is it safe to keep resetting a Biasi boiler showing E04?
Resetting the boiler one or two times to see whether a one-off pressure dip clears the fault is reasonable. But repeatedly resetting without addressing the root cause is not a good idea. The boiler locks out on E04 to protect the pump and heat exchanger from running without adequate water circulation — continuing to reset it can cause those components to overheat and fail, turning a relatively inexpensive repair into a much costlier one. If the fault returns after a couple of resets, call a Gas Safe registered engineer.