Biasi E09 Fault Code: Causes, Fixes & Repair Costs
What does the Biasi E09 fault code mean?
The Biasi E09 (also displayed as ER 09 on some models) fault code indicates that the boiler's control board has detected a problem with the flue temperature NTC probe — a thermistor sensor that monitors the temperature of gases leaving through the flue. When this sensor gives a reading outside expected parameters, or stops responding altogether, the boiler locks out as a safety precaution and will not restart until the fault is resolved. Note: a small number of Biasi models use E09 to flag a water pressure error rather than a flue probe fault — always cross-reference with your specific model's manual if you are unsure which applies to you.
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
- Failed or degraded flue NTC temperature probe Common
The flue temperature sensor itself is the most frequent culprit. NTC thermistors degrade over time and can fail outright, sending readings to the PCB that fall well outside the normal operating range. The boiler interprets this as a safety risk and locks out. Replacing the probe is the first repair a Gas Safe engineer will attempt.
- Damaged or corroded wiring to the probe Sometimes
Even if the probe itself is fine, the wiring harness connecting it to the PCB can suffer from heat stress, moisture ingress, or simple wear. Corroded terminals or a broken wire will produce erratic or absent signals that look identical to a probe failure. An engineer will inspect and test the circuit before condemning the sensor.
- Flue blockage causing abnormal flue temperatures Sometimes
A partial blockage in the flue — whether from debris, a bird's nest, or a sagging flue joint — can cause flue gas temperatures to rise or behave irregularly, triggering the probe's safety threshold. Clearing the obstruction resolves the code without any parts being replaced.
- Low system pressure (select models only) Sometimes
On certain Biasi models, E09 is associated with a water pressure fault rather than the flue probe. If your boiler's pressure gauge reads below 1 bar, this may be the cause. Check your model's documentation to confirm which interpretation applies before spending money on sensor diagnostics.
- Faulty PCB (control board) Rare
If a new probe and sound wiring still fail to clear the fault, the PCB itself may be misreading the sensor signal or has developed an internal fault. PCB replacement is a last resort, typically explored only after other components have been ruled out.
How to fix it
- Check the system pressure gauge DIY safe
Find the pressure gauge on the front or underside of your boiler. It should read between 1 and 1.5 bar when cold. If it reads below 1 bar and your model links E09 to a pressure fault, use the filling loop to top the system up to around 1.2–1.5 bar, following the instructions in your boiler's user guide. Do not overfill beyond 2 bar.
- Attempt a single boiler reset DIY safe
Press and hold the reset button on your Biasi boiler as described in your user manual — typically for around 3 seconds — then wait for the boiler to attempt a restart. If the E09 code reappears immediately or returns within a short time, the underlying fault is still present and resetting again will not fix it. Limit yourself to one or two resets at most.
- Check that the flue terminal outside is clear DIY safe
Take a look at the flue outlet on the outside wall of your property. Make sure it is not obstructed by debris, a bird's nest, overgrown vegetation, or anything that could restrict the flow of flue gases. Do not insert anything into the flue or attempt to dismantle any flue components — simply observe from outside and report your findings to the engineer.
- Call a Gas Safe registered engineer to diagnose and replace the flue NTC probe Gas Safe engineer
An engineer will use a multimeter to test the resistance of the flue temperature NTC probe against the manufacturer's specifications. If the probe is out of range or open-circuit, it will be replaced with a compatible part. This is the most common resolution for E09 and is not a task a homeowner should attempt — the probe is mounted within the flue assembly and requires safe access and correct calibration.
- Have the engineer inspect wiring and connections Gas Safe engineer
If a new probe does not clear the fault, the engineer should trace the wiring loom from the probe back to the PCB, checking for corroded terminals, heat damage, or broken conductors. Repairing or replacing the wiring harness is straightforward for a qualified engineer but involves working inside the boiler casing.
- Have the engineer inspect the flue system thoroughly Gas Safe engineer
If no probe or wiring fault is found, the engineer should check the entire flue run for partial blockages, poorly seated joints, or sections that may be causing irregular flue temperatures. Any obstruction or flue integrity issue must be corrected before the boiler is returned to service.
- PCB testing and replacement if all else fails Gas Safe engineer
As a last resort, the engineer may test the PCB's signal input circuitry. If the board is confirmed faulty, it will need to be replaced. Given the cost of a PCB, a reputable engineer will exhaust cheaper possibilities first and may advise you on whether repair is economical given the age of the boiler.
Parts you may need
- Flue temperature NTC probe / thermistor · from £35
- Wiring harness / loom (probe to PCB) · from £25
- PCB (control 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–£350, depending on the underlying cause.
Frequently asked questions
Is the Biasi E09 code always a flue probe fault, or can it mean something else?
The official Biasi Inovia ERP service manual identifies E09 (ER 09) as a faulty flue temperature NTC probe. However, at least one source associates the code with a water pressure fault on certain models. The safest approach is to check your specific model's manual — if you have lost it, Biasi customer support or ManualsLib can help you find the correct documentation. An engineer visiting your home will test both possibilities before ordering parts.
Can I reset a Biasi E09 fault myself and will it fix the problem?
You can attempt a reset — press and hold the reset button for a few seconds as described in your user guide. Occasionally a one-off sensor glitch will clear after a reset. However, if the E09 code returns, the underlying fault is still present and further resets will not resolve it. Limit yourself to one or two attempts, then call a Gas Safe registered engineer to diagnose the root cause.
How much does it cost to fix a Biasi E09 fault in the UK?
For the most common repair — replacing the flue NTC probe including parts and labour — most homeowners pay in the region of £150 to £250. If the fault turns out to be wiring-related, the cost is similar. If the PCB needs replacement, the bill can rise to £350–£500 or more, though engineers will only recommend this after ruling out cheaper fixes. These figures are for standard weekday appointments; emergency or out-of-hours call-outs typically add £100 or more to the total.
My Biasi boiler is still under warranty — will E09 be covered?
Biasi offer warranties of between 5 and 7 years depending on the model. If your boiler is within its warranty period and has been annually serviced by a Gas Safe engineer as required by the warranty terms, contact Biasi directly before arranging an independent repair. A covered fault could save you the full cost of parts and labour. Keep your service records to hand when you call.