Biasi E05 Fault Code: Causes, Fixes & Repair Costs
What does the Biasi E05 fault code mean?
The E05 (also displayed as ER05 on some Biasi models) fault code indicates that the boiler has locked out due to a problem with the fan or the draught pressure circuit. When your Biasi boiler starts up, the PCB (printed circuit board) checks that the fan is spinning correctly and generating enough airflow before it allows ignition. It does this by monitoring the air pressure switch, which should detect a specific pressure change as the fan ramps up. If that pressure change is absent, insufficient, or irregular, the PCB treats this as unsafe — waste combustion gases could otherwise build up inside the flue — and immediately locks the boiler out. The E05 code is the boiler's way of telling you it could not confirm safe draught conditions at start-up.
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 seized fan Common
The fan motor may have failed outright or become mechanically jammed. If the fan cannot spin, no draught is created, the air pressure switch registers no pressure change, and the PCB locks the boiler out immediately. This is the single most frequent cause of an E05 code.
- Blocked or partially obstructed flue Common
If debris, a bird's nest, or external blockage restricts the flue outlet, the fan struggles to push waste gases out even when running normally. The resulting back-pressure confuses the air pressure switch and can trigger an E05 lockout.
- Faulty air pressure switch Sometimes
The air pressure switch (sometimes called the draught or thrust sensor) is the component that feeds draught information back to the PCB. If the switch itself has failed or its connecting rubber hose has cracked or become disconnected, it will report incorrect pressure readings regardless of whether the fan is working fine.
- Loose or damaged wiring and electrical connections Sometimes
The fan and pressure switch both rely on clean electrical connections to the PCB. Corrosion, a loose plug, or damaged wiring can cause intermittent or absent signals, making the PCB believe the fan has failed even when it is physically intact.
- Fan running too slowly Sometimes
In some cases the fan spins but not fast enough to generate the required draught. This can be caused by a worn motor, a partial blockage in the venturi pipe, or a PCB that is not supplying the correct voltage to the fan.
- Water damage to the fan or surrounding components Rare
Internal leaks — often originating from the heat exchanger or pump — can allow moisture to reach the fan motor or the PCB connections serving it. Water damage tends to cause gradual degradation rather than sudden failure, but it will eventually trigger an E05 lockout and can spread to other expensive components if left unaddressed.
How to fix it
- Turn the boiler off and check for obvious flue obstructions outside DIY safe
Go outside and visually inspect the flue terminal (the white pipe exiting through an external wall or roof). Check for debris, leaves, a bird's nest, or ice blocking the outlet. Do not probe inside the flue or dismantle anything — this visual check is all a homeowner should do.
- Attempt a single boiler reset DIY safe
Press and hold the reset button (usually marked with a flame or 'R' symbol) for roughly 3 seconds as described in your user manual. Wait for the boiler to complete its start-up sequence. If it fires and then locks out again within minutes, do not reset it repeatedly — the underlying fault has not gone away. Repeated resets on a fan-related lockout will not fix the problem and could in theory allow unsafe conditions to develop momentarily.
- Do not attempt to access or test the fan yourself Gas Safe engineer
The fan is an internal gas-appliance component. Accessing it requires removing boiler casing and working in the vicinity of the heat exchanger and gas components. This must only be carried out by a Gas Safe registered engineer. A homeowner attempting this could void any remaining warranty and create a gas safety hazard.
- Engineer to inspect wiring, connections, and fan speed Gas Safe engineer
A Gas Safe engineer will remove the boiler casing and check all wiring looms and plug connectors to the fan and air pressure switch. They will use a multimeter to verify that the fan is receiving the correct voltage from the PCB and that the motor's resistance readings are within specification. They will also check whether the fan is running at the correct speed.
- Engineer to inspect and test the air pressure switch and its hose Gas Safe engineer
The engineer will check the rubber hose connecting the fan housing to the air pressure switch for cracks, kinks, or disconnection, and will test the switch itself. A faulty hose can often be replaced cheaply; a failed switch will need to be swapped out.
- Engineer to replace the fan if it has failed Gas Safe engineer
If the fan motor has seized, burned out, or is running too slowly to bring up to specification, a direct replacement fan for the specific Biasi model will be fitted. The engineer should also check the venturi pipe and fan housing for any partial blockages that may have contributed to the failure.
- Engineer to investigate and resolve any internal water damage Gas Safe engineer
If water damage is found on or around the fan, the engineer must trace the source — commonly a leaking heat exchanger or pump seal — and repair it before fitting any new components. Replacing the fan without fixing the leak will result in the same fault recurring and risks further damage to the PCB and other electronics.
- Call a Gas Safe registered engineer if the boiler has not responded to a single reset Gas Safe engineer
Given that the E05 code relates to a safety-critical draught system, any fault that persists after one reset must be diagnosed by a Gas Safe registered engineer. Do not leave the boiler running or continue resetting it. Book an engineer promptly, particularly in cold weather.
Parts you may need
- Biasi boiler fan / fan motor assembly · from £120
- Air pressure switch (draught sensor) · from £35
- Pressure switch connecting hose / tube · from £8
- Fan wiring loom / connector plug · from £20
- PCB (printed circuit board) — if fan fault has caused board damage · 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 £150–£380, depending on the underlying cause.
Frequently asked questions
Can I reset my Biasi E05 fault myself and will it fix the problem?
You can try a single reset, and occasionally a one-off glitch will clear this way. However, if the boiler locks out again with E05, resetting repeatedly will not resolve the underlying fault. The E05 code means the boiler's safety system could not confirm safe draught conditions, so continuing to override it is not advisable. One reset attempt is reasonable; anything beyond that needs a Gas Safe engineer.
How much does it cost to fix a Biasi E05 fault in the UK?
Most people with an E05 code end up paying somewhere between £150 and £380 all-in, depending on what needs replacing. A straightforward fan replacement — the most common repair — typically costs around £220–£280 including labour. If the air pressure switch or its hose is the culprit, the bill is usually lower. If the engineer discovers internal water damage alongside a failed fan, or if the PCB has been affected, costs can rise above £400. At that point it is worth discussing with your engineer whether a new boiler represents better value, especially if your Biasi is more than eight to ten years old.
Is the Biasi E05 fault dangerous — should I stop using the boiler?
Yes, treat the boiler as unsafe to operate until it has been inspected. The E05 code exists precisely because the boiler could not verify that harmful flue gases would be safely expelled. The boiler locks itself out as a precaution, which is the correct response. Leave it switched off and do not attempt to keep resetting it to get heat. Call a Gas Safe engineer as soon as possible.
My Biasi boiler is still under warranty — does E05 mean it will be covered?
Biasi offers warranties of five to seven years depending on the model, though warranty terms typically require annual professional servicing to remain valid. If your boiler is within its warranty period and has been serviced as required, contact Biasi directly before booking an independent engineer, as the repair may be covered at no cost or reduced cost. Have your installation documents and service records to hand when you call.