Biasi ER08 Fault Code: Causes, Fixes & Repair Costs
What does the Biasi ER08 fault code mean?
The ER08 code means your Biasi boiler has detected a problem with the fan or flue system and has shut itself down as a safety precaution. The boiler's internal sensors have determined that combustion gases — which can include carbon monoxide — are not being safely vented away from the appliance. Rather than continue operating under potentially dangerous conditions, the boiler locks out and displays ER08. Because this fault relates directly to the removal of harmful gases, it must be treated seriously and should not be reset repeatedly without investigating the underlying cause.
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 boiler fan Common
The fan is responsible for drawing combustion gases through the heat exchanger and pushing them safely out through the flue. If the fan motor has seized, is running too slowly, or has failed altogether, the boiler cannot safely expel exhaust gases. The PCB monitors fan performance and will trigger ER08 as soon as it detects a problem. A failing fan is the single most common cause of this code on Biasi boilers.
- Blocked or obstructed flue Common
If the flue terminal on the outside wall is partially or fully blocked — by a bird's nest, leaves, ice, or other debris — exhaust gases cannot escape freely. This causes back-pressure in the system, which the boiler interprets as a fan or venting failure. Flue blockages are especially common during autumn and winter and after prolonged periods of the boiler being switched off.
- Faulty air pressure switch Sometimes
The air pressure switch monitors whether the fan is generating sufficient airflow before and during a heating cycle. If the switch has failed, is stuck in the open position, or has a cracked or detached rubber hose connected to it, it cannot confirm that the fan is working — even if the fan itself is fine. This causes a false ER08 lockout. The switch is a relatively common wear item on older Biasi boilers.
- Loose wiring or poor electrical connections Sometimes
The fan, air pressure switch, and PCB are all linked by wiring looms and connectors. Vibration over time can loosen these connections, causing intermittent or absent signals. The PCB may then mistakenly conclude that the fan has failed when the wiring is actually the culprit. This tends to produce an ER08 code that appears and clears inconsistently.
- PCB fault Rare
The printed circuit board controls and monitors the fan circuit. If the PCB itself has developed a fault — such as a failed relay or damaged component — it may be unable to correctly command or read the fan, resulting in an ER08 lockout even when all other components are working correctly. PCB faults are less common but should be considered if an engineer has ruled out all other causes.
How to fix it
- Listen and feel for fan operation DIY safe
Stand close to the boiler casing. The fan typically starts before the burner ignites. Listen for a gentle hum and, if safe to touch the casing, feel for slight vibration. If you can detect neither, the fan may have stopped entirely. This observation is useful information to pass on to your engineer but does not change the need to call one.
- Check the flue terminal outside DIY safe
Go outside and locate the flue outlet on the external wall (usually a white plastic pipe or twin-pipe terminal). Check visually that it is not blocked by a bird's nest, ice, leaves, or anything else. Do not poke anything inside the flue terminal. If you can see an obvious external blockage that can be safely removed without touching the flue pipe itself, you may carefully clear it, but if in any doubt leave it for the engineer.
- Attempt a single boiler reset DIY safe
Locate the reset button or reset procedure for your specific Biasi model (refer to your user manual). Press and hold the reset button until the lockout clears, then allow the boiler to attempt a restart. Only do this once or twice at most. If the boiler locks out again with ER08, do not keep resetting it — repeated resets on a fan or flue fault can mask a genuine safety issue.
- Call a Gas Safe registered engineer to inspect the fan and pressure switch Gas Safe engineer
A qualified engineer will use a multimeter to test the fan motor's electrical supply and output, check whether the fan is running at the correct speed, and inspect the air pressure switch and its connecting hoses for cracks or blockages. They can adjust fan speed settings where the boiler allows, or replace the fan or pressure switch as required.
- Engineer to inspect the flue system internally Gas Safe engineer
The engineer will carry out a full internal inspection of the flue connections within the boiler casing and trace the flue run for any signs of separation, blockage, or incorrect installation. Any section of the flue that has come apart must be correctly resealed before the boiler is returned to service.
- Engineer to check wiring and PCB if other components test satisfactory Gas Safe engineer
If the fan and pressure switch are both functioning correctly and the flue is clear, the engineer will inspect the wiring harness and individual connectors for signs of corrosion, burning, or looseness. If wiring is sound, the PCB becomes the prime suspect and will require specialist testing or replacement.
Parts you may need
- Boiler fan / flue fan assembly (Biasi compatible) · from £120
- Air pressure switch · from £35
- Pressure switch connecting hose / tubing · from £8
- 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–£380, depending on the underlying cause.
Frequently asked questions
Is ER08 dangerous — should I stay in the house?
The ER08 lockout exists precisely to protect you: the boiler has stopped itself from running because it cannot confirm that combustion gases are being safely removed. As long as the boiler remains locked out and is not running, there is no ongoing production of gases such as carbon monoxide. However, you should treat the fault seriously, ensure your carbon monoxide alarm is functioning (fit one if you do not have one), ventilate the room if you smell anything unusual, and arrange for a Gas Safe engineer to attend promptly. Do not repeatedly reset the boiler in an attempt to keep it running.
Can I fix ER08 myself to save money?
The basic checks you can safely carry out yourself are limited to visually inspecting the outside flue terminal for obvious blockages and attempting a single reset. Everything else — testing the fan, replacing the air pressure switch, inspecting the internal flue connections, or touching any wiring — must be done by a Gas Safe registered engineer. Working on the fan or flue system without the correct qualifications is both illegal and dangerous.
How much does it typically cost to fix ER08 on a Biasi boiler?
For most people, the repair involves either a fan service or replacement, or a new air pressure switch, combined with a call-out charge. You should budget roughly £150–£380 for these scenarios. A fan replacement on a Biasi typically falls in the £250–£300 range including labour. An air pressure switch is cheaper to replace. If the fault turns out to be the PCB, costs can exceed £500 including parts and labour — at that point many engineers and boiler advisers suggest comparing the cost of a new boiler, particularly if the appliance is already over eight to ten years old.
My Biasi boiler is still under warranty — does ER08 affect my cover?
Biasi boilers typically come with a manufacturer's warranty of five to seven years depending on the model and registration. If your boiler is within this period, contact Biasi directly before booking an independent engineer, as an unauthorised repair could affect your warranty terms. Have your model number, serial number, and installation date to hand when you call. If the warranty has lapsed, use any Gas Safe registered heating engineer you trust.