Biasi ER25 Fault Code: Causes, Fixes & Repair Costs
What does the Biasi ER25 fault code mean?
The ER25 code appears on Biasi boilers when the control board cannot confirm a stable flame during or after the ignition sequence. Every time your boiler fires up, the PCB orchestrates a precise sequence: the fan spins up, the air pressure switch confirms adequate flue draught, the gas valve opens, and the ignition electrode sparks. A dedicated ionisation sensor then passes a tiny electrical current through the flame to verify it is burning correctly. If that confirmation signal never arrives — or arrives when it should not — the boiler locks out and displays ER25 as a safety precaution. The fault most commonly appears when the heating is first called for, leaving you with no hot water or central heating until the underlying problem is identified and resolved.
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
- Faulty or dirty ionisation/flame sensor electrode Common
The ionisation sensor generates a microamp signal when it sits inside a burning flame. Over time the ceramic body can crack, the electrode tip can corrode, or a layer of oxidisation can build up on the tip — all of which reduce or eliminate the signal. A Gas Safe engineer will measure the DC microamp output during a live ignition attempt; a healthy sensor typically reads between 0.5 µA and 5 µA. No reading, or a reading outside that range, points squarely at the sensor. Because the sensor is relatively inexpensive and simpler to swap than other components, it is usually tested first.
- Gas valve fault Common
If the gas valve fails to open fully — or opens unevenly — the burner receives insufficient gas to sustain ignition. The PCB registers no stable flame and locks the boiler out. Some homeowners report a faint ringing or vibrating noise from the boiler just before lockout, which can be an indicator of irregular gas flow through a partially seized valve. Gas valve faults present almost identically to sensor faults from the outside, so professional diagnosis is essential to distinguish between them.
- PCB (printed circuit board) fault Sometimes
The PCB contains the flame detection circuitry that interprets the ionisation sensor signal. If the detection circuit on the board develops a fault, it can fail to register a perfectly healthy flame — giving a false ER25 lockout. PCB faults can also cause intermittent rather than constant ER25 appearances. A qualified engineer will rule out the sensor and gas valve first, since both are cheaper and more commonly at fault, before concluding the PCB is to blame.
- Faulty ignition lead or wiring issue Sometimes
The cable running from the PCB to the ionisation sensor is a high-impedance circuit — any damage to the insulation, a loose connection at either end, or a short to earth will corrupt the flame signal. Similarly, damaged ignition leads can cause unreliable sparking, meaning a flame is never properly established in the first place. A visual inspection of leads and connections is a straightforward early check for an engineer.
- Diverter valve fault Rare
On combination boilers, a diverter valve that is stuck or slow to move can interfere with correct water flow during the firing sequence, affecting combustion conditions. While it is a less direct cause of flame detection errors than the sensor or gas valve, a failing diverter valve is known to trigger ER25 on some Biasi models — particularly if the code returns after a successful reset. A faulty diverter valve will also typically disrupt either heating or hot water delivery.
How to fix it
- Press the reset button once (hold for approximately 10 seconds) DIY safe
Locate the reset button on the boiler fascia — it is usually marked with a flame or reset symbol. Press and hold for around 10 seconds until the boiler attempts to restart. You may hear the fan spin up and an ignition click. If the boiler fires and runs normally, monitor it over the next day or two. If ER25 returns, do not keep resetting — more than two or three resets in a row risks flooding the combustion chamber with unburned gas, which is dangerous.
- Check the boiler system pressure DIY safe
Look at the pressure gauge on the boiler front panel. It should read between 1 and 1.5 bar when the system is cold. If it has dropped below 1 bar, use the filling loop (the small braided hose or valve beneath or near the boiler) to slowly top up the pressure until the gauge reads around 1.2–1.5 bar, then close the filling loop valve. Low pressure alone is unlikely to cause ER25, but it is worth ruling out before calling an engineer.
- Confirm your gas supply is working DIY safe
Check that other gas appliances in your home — a gas hob or gas fire — are functioning normally. If they are not, your gas supply may be interrupted. Check that the gas isolation valve on the pipe entering your boiler has not been accidentally turned off (the handle should be parallel to the pipe, not at a right angle). If you suspect a gas supply problem, contact your gas network provider rather than your boiler engineer.
- Do not attempt to access internal components Gas Safe engineer
The ionisation sensor, ignition electrode, gas valve, PCB, and all associated wiring are inside the boiler casing and must only be accessed by a Gas Safe registered engineer. Attempting to inspect or remove these components yourself is illegal for gas-related work and potentially fatal. Even if you have some DIY experience, working on gas appliances without Gas Safe registration is a criminal offence in the UK.
- Call a Gas Safe registered engineer to diagnose and repair the fault Gas Safe engineer
A qualified engineer will carry out a structured diagnostic: measuring the ionisation current during ignition, checking the ignition leads and wiring for continuity, testing gas valve operation and output, and inspecting the PCB flame detection circuit if all other components check out. Depending on findings, they will clean or replace the ionisation sensor, replace the gas valve, repair or replace the PCB, or address the diverter valve. Always ask to see the engineer's Gas Safe ID card before work begins — you can verify credentials at the Gas Safe Register website.
Parts you may need
- Ionisation/flame sensor electrode · from £35
- Ignition lead set · from £25
- Gas valve · from £120
- Diverter valve · from £75
- PCB (printed circuit board) · from £220
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 £120–£380, depending on the underlying cause.
Frequently asked questions
Can I reset my Biasi boiler myself to clear the ER25 code?
Yes, a single reset is a reasonable first step and is safe for a homeowner to do. Press and hold the reset button for about 10 seconds and see whether the boiler fires successfully. If it does, keep an eye on it — a one-off lockout can sometimes be caused by a momentary gas supply blip or a brief ignition failure. However, if ER25 returns after one or two resets, stop resetting and call a Gas Safe engineer. Repeatedly resetting a boiler with an active fault can push unburned gas into the combustion chamber, creating a safety hazard.
How much does it cost to fix a Biasi ER25 fault in the UK?
Most homeowners pay somewhere between £120 and £380 all-in, depending on which component is at fault. Cleaning or replacing the ionisation sensor typically falls at the lower end (around £120–£220 including labour), while a gas valve replacement usually runs £200–£350. A PCB repair or replacement is the most expensive common fix and can reach £300–£600 or more depending on parts availability for your specific Biasi model — if your boiler is older and the PCB is hard to source, a new boiler may be more cost-effective. Always get at least two quotes from Gas Safe engineers before committing to a major repair.
My Biasi warranty is still valid — does ER25 affect my cover?
Biasi boilers typically come with a 5 to 7 year warranty depending on the model and registration. If your boiler is within that period, contact Biasi or your installer before paying for an independent repair, as the work may be covered at no cost. Be aware that warranty cover can be voided if the boiler has not been serviced annually by a Gas Safe engineer, so make sure you have a service record to hand when you make the call.
Could ER25 mean my boiler needs replacing rather than repairing?
Not necessarily — in most cases ER25 is caused by a sensor or gas valve fault that is economical to fix. As a rough rule of thumb, if the repair quote is under £500 and the boiler is fewer than 10 years old, repair usually makes financial sense. If the PCB needs replacing on a boiler that is 12 or more years old and out of warranty, the maths can tip towards replacement — particularly given the improved efficiency of modern A-rated condensing boilers. Ask your Gas Safe engineer for an honest assessment of the boiler's overall condition before committing to an expensive part.