Biasi E154 Fault Code: Causes, Fixes & Repair Costs
What does the Biasi E154 fault code mean?
The E154 fault code (sometimes also displayed as A04 on certain Biasi models) indicates that the boiler's control board has not received confirmation that the fan is running correctly. Biasi boilers use an air pressure switch (APS) — a safety device connected to the fan via heat-resistant venturi tubes — to detect whether the fan is generating sufficient airflow. When the fan spins, the resulting air movement closes the APS, sending a signal to the PCB that ventilation is active and combustion gases can be safely expelled through the flue. If the boiler cannot confirm this — whether due to a failed fan, a blocked flue, a faulty APS, or a wiring issue — it locks out and displays E154 to protect the occupants from potentially harmful gases accumulating indoors.
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
- Blocked or obstructed flue terminal Common
Debris such as leaves, a wasp or bird nest, or compacted snow around the external flue outlet can restrict the airflow the fan needs to create. The APS detects the reduced pressure differential and triggers a lockout. This is one of the most straightforward causes to check from outside your property.
- Fan failure or seized fan motor Common
If the fan motor has burned out, seized, or its bearings have worn, it will not spin fast enough — or at all — to close the air pressure switch. The boiler then locks out before any gas is released to the burner. A Gas Safe engineer can confirm this with a multimeter and visual inspection.
- Faulty air pressure switch Sometimes
The APS itself can fail over time. Its internal diaphragm can crack or become stuck, meaning it no longer responds correctly to the airflow the fan produces — even when the fan is working perfectly. If the engineer rules out a fan or flue issue, a faulty APS is the next most likely culprit.
- Blocked or kinked venturi tubes Sometimes
The small silicone or rubber tubes connecting the fan's venturi ports to the air pressure switch can become blocked with debris, condensation residue, or kink under compression. A partial blockage is enough to give the APS a false low-pressure reading.
- Loose or damaged wiring connections Sometimes
Wiring between the fan, the air pressure switch, and the PCB can loosen over time due to vibration or heat cycling. A poor connection can interrupt the signal the PCB needs, causing an E154 even when all components are mechanically sound.
- Faulty PCB Rare
In rare cases, when every other component tests correctly, the PCB itself may be misreading or failing to process the signal from the APS. This is a last-resort diagnosis and typically makes an engineer consider whether a full boiler replacement is more cost-effective.
How to fix it
- Check the external flue terminal for visible blockages DIY safe
Go outside and look at the point where the flue exits the building — usually a plastic terminal on an external wall or through a roof. Remove any obvious debris such as leaves, spiders' webs, or accumulated dirt by hand. Do not use ladders if you are not confident doing so safely. If there is a wasp or bird nest, you may need a pest-control visit before the engineer attends.
- Listen and feel for fan operation DIY safe
Place your hand gently near the boiler casing (not on any hot surfaces) and listen carefully. A working fan produces a low hum and a faint vibration. If you can hear and feel nothing when the boiler attempts to start, this strongly suggests the fan is not running and you should move to calling an engineer. This is an observational check only — do not open the boiler casing.
- Reset the boiler — once or twice at most DIY safe
Locate the reset button on the Biasi front panel (often marked with a flame or reset symbol). Press and hold it for approximately 5–10 seconds until the display changes or the boiler begins its start-up sequence. Wait up to 10 minutes to see if the boiler fires successfully. If it locks out again with E154, attempt a second reset. Do not reset more than twice in succession — repeated failed attempts serve no purpose and should not be used as a substitute for diagnosis.
- Check your boiler's warranty status DIY safe
Biasi boilers typically carry a manufacturer's warranty of five to seven years depending on the model and registration. If your boiler is still within warranty, contact Biasi directly before arranging an independent engineer, as covered repairs could be carried out at no cost to you.
- Call a Gas Safe registered engineer to inspect the fan, APS, and venturi tubes Gas Safe engineer
An engineer will use a multimeter to test whether the fan motor is drawing the correct current and producing the right rotational speed. They will also test the air pressure switch for continuity in both open and closed states, and inspect the venturi tubes for blockages or damage. These checks require the boiler casing to be removed and live components to be tested — this must only be done by a Gas Safe registered professional.
- Engineer to replace faulty components as identified Gas Safe engineer
Depending on the diagnosis, the engineer will replace the fan motor, air pressure switch, venturi tubes, or wiring harness as required. Using genuine Biasi parts or quality approved equivalents is recommended to avoid repeat failures. After replacement, the boiler will be tested through a full ignition cycle to confirm the E154 has cleared.
- PCB inspection and replacement if all other components are confirmed healthy Gas Safe engineer
If the fan, APS, wiring, flue, and venturi tubes all test correctly, the engineer may conclude the PCB is at fault. A PCB replacement is a significant cost — at this point it is worth asking the engineer for an honest assessment of whether repair or full boiler replacement represents better long-term value.
Parts you may need
- Air pressure switch (APS) · from £45
- Boiler fan motor assembly · from £120
- Venturi tubes / APS connection tubing · from £15
- Fan wiring harness · from £25
- 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 £150–£400, depending on the underlying cause.
Frequently asked questions
Can I fix Biasi E154 myself without an engineer?
Some preliminary checks are safe to carry out yourself: looking for a blocked flue terminal outside, listening for the fan, and attempting one or two resets. However, any internal inspection or component replacement — including the fan, air pressure switch, venturi tubes, or PCB — must be performed by a Gas Safe registered engineer. Working on the internal components of a gas appliance without Gas Safe registration is illegal and dangerous.
How much does it cost to fix a Biasi E154 fault in the UK?
Most homeowners pay between £150 and £400 to resolve an E154, depending on the faulty component. An air pressure switch replacement typically costs £120–£200 including labour, while a fan motor replacement usually runs £220–£380. Call-out and diagnostic fees in 2025 are generally £100–£150. In the worst case, if the PCB is at fault, costs can exceed £500 — at which point a new boiler may be a more sensible investment, especially if the boiler is over ten years old.
Why does my Biasi boiler keep showing E154 after I reset it?
If the boiler relocks out with E154 shortly after each reset, the underlying cause has not gone away. A reset simply allows the boiler to attempt another start; it does not fix the fault. Persistent lockouts strongly suggest a failed fan motor, a faulty air pressure switch, or a blocked flue that needs professional attention. Avoid repeatedly resetting the boiler as this will not resolve the issue and wastes time before getting help.
What is the difference between the E154 and A04 codes on a Biasi boiler?
Both codes point to the same underlying problem — a failure to confirm correct fan or air pressure switch operation. A04 appears on some older Biasi display panels and firmware versions, while E154 is the code used on more recent models and documentation. If you see either on your boiler, the diagnosis and repair process is identical: check the flue, attempt a reset, and call a Gas Safe engineer if the lockout persists.