Biasi ER05 Fault Code: Causes, Fixes & Repair Costs
What does the Biasi ER05 fault code mean?
The ER05 code indicates that your Biasi boiler has detected a problem with the fan and has locked itself out as a safety measure. The fan plays a critical role in safe boiler operation — it creates the draught that pushes harmful combustion gases out through the flue before they can accumulate. Before allowing ignition, the PCB (the boiler's control board) checks that the fan is running correctly by monitoring the air pressure switch. If the air pressure change on start-up isn't what the PCB expects — because the fan is seized, running too slowly, or not responding at all — it locks the boiler out and displays ER05. This is a deliberate safety lockout, not simply an error to reset away.
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
- Fan seized or mechanically jammed Common
The fan impeller can seize over time due to wear, corrosion, or debris. A completely stationary fan produces no air pressure change, so the air pressure switch immediately signals to the PCB that it is unsafe to fire the boiler — resulting in an ER05 lockout.
- Fan running below required speed Common
Even if the fan spins, a speed that is too low will not generate sufficient draught to safely expel flue gases. The air pressure switch detects that the pressure differential is inadequate and reports this back to the PCB, triggering the lockout before combustion begins.
- Wiring fault or loose electrical connections Common
Corroded terminals, loose plugs, or damaged wiring between the PCB and the fan can cause intermittent or absent signals. The PCB may conclude the fan is non-functional even when it is mechanically sound, leading to a spurious but genuine ER05 lockout.
- Fan unit has failed completely Sometimes
If wiring and fan speed have both been checked and found to be acceptable, the fan itself may have an internal electrical failure. A Gas Safe engineer can confirm this with a multimeter and will advise on replacement.
- Water ingress or moisture damage to the fan Sometimes
Condensate or a leak from a nearby component can introduce moisture into the fan motor or its electrical connections, accelerating failure. If water damage is the root cause, any replacement fan is at risk of the same fate unless the source of ingress is identified and fixed at the same time.
- Faulty air pressure switch Sometimes
Rather than the fan itself being at fault, the air pressure switch that monitors fan output may have failed or its connecting hose may have split or blocked. This means the switch sends an incorrect 'no pressure' signal to the PCB even when the fan is working perfectly.
How to fix it
- Turn the boiler off at the programmer or room thermostat and leave it switched off DIY safe
Do not attempt to reset the boiler repeatedly. Unlike some other fault codes, ER05 relates to a safety-critical component. Repeatedly resetting and running the boiler while this fault is present could be dangerous. Turn the boiler off and wait for a Gas Safe engineer.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in your home (such as your hob or gas fire) are working normally. If they are not, contact your gas supplier rather than a boiler engineer, as the issue is upstream of the boiler.
- Do not attempt to access, move, or inspect the fan yourself Gas Safe engineer
The fan is located inside the boiler casing and accessing it requires removal of sealed components. This work must only be carried out by a Gas Safe registered engineer. Do not remove the boiler casing or attempt to spin the fan manually.
- A Gas Safe engineer will inspect and test the fan Gas Safe engineer
The engineer will remove the boiler casing and check whether the fan spins freely or has seized. They will also observe whether it runs at the correct speed on start-up and listen for any unusual noise indicating mechanical wear.
- A Gas Safe engineer will test wiring and electrical connections with a multimeter Gas Safe engineer
Not all wiring faults are visible to the naked eye. The engineer will use a multimeter to check continuity and voltage across the fan's wiring harness, terminals, and connections to the PCB. Any loose or corroded connections will be re-made or replaced.
- A Gas Safe engineer will test the air pressure switch and its hose Gas Safe engineer
The engineer will check the rubber hose connecting the fan outlet to the air pressure switch for splits, blockages, or disconnection. The switch itself will be tested to confirm it is producing the correct signal at the correct pressure differential.
- A Gas Safe engineer will clean the flue if needed Gas Safe engineer
A partially blocked flue can restrict air flow sufficiently to prevent the fan from building adequate pressure. The engineer will inspect the flue termination for obstructions such as bird nests, debris, or ice in cold weather.
- If the fan requires replacement, a Gas Safe engineer will fit a new unit Gas Safe engineer
Where the fan has failed mechanically or electrically, the engineer will source and fit a compatible replacement Biasi fan. They will also investigate whether moisture ingress contributed to the failure and address that at the same time.
- Once the fault is resolved, the engineer will reset the boiler to clear the ER05 code Gas Safe engineer
After the underlying cause has been identified and fixed, the engineer will perform the reset. If the fault reappears shortly after reset, further diagnostics are needed — do not continue resetting without a confirmed repair.
Parts you may need
- Biasi boiler fan/flue fan assembly · from £95
- Air pressure switch · from £35
- Air pressure switch hose · from £8
- Fan wiring harness/connector · 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 £150–£380, depending on the underlying cause.
Frequently asked questions
Can I reset the boiler myself to clear the ER05 code?
It is best not to. Some lockout codes are safe to reset once or twice while you arrange an engineer visit, but ER05 relates directly to a component that keeps harmful flue gases out of your home. If the fan is not working correctly, running the boiler — even briefly — is not advisable. Leave the boiler off and call a Gas Safe registered engineer promptly.
How much does it cost to fix a Biasi ER05 fan fault in the UK?
For most homeowners the total bill, including labour and parts, sits somewhere between £150 and £380. A straightforward fix such as remaking a loose wiring connection or replacing the air pressure switch hose will sit towards the lower end. A full fan replacement, which is the most common repair for this fault, typically comes out at around £200–£300 all in. In rare cases where the PCB has also been damaged by a failing fan, costs can climb higher — but that scenario is uncommon and would be discussed with you before any work is undertaken.
Is my Biasi boiler still under warranty?
Biasi offers a warranty of between five and seven years on most models, covering faults caused by defective parts or workmanship. To be eligible, the boiler must have been registered with Biasi within 30 days of installation. If you are within the warranty period, contact Biasi directly before booking an independent engineer, as a warranty repair may be free of charge or substantially reduced in cost.
Could a blocked flue cause the ER05 code?
Yes, this is worth considering — especially if the fault appeared after a period of cold weather or if the flue outlet is in a location where debris or bird nests could accumulate. A restricted flue means the fan cannot build sufficient pressure even if it is working perfectly, and the air pressure switch will correctly report this to the PCB. A Gas Safe engineer will inspect the flue as part of the diagnostic process.