Biasi ER09 Fault Code: Causes, Fixes & Repair Costs
What does the Biasi ER09 fault code mean?
The ER09 code appears when your Biasi boiler detects an abnormal or out-of-range signal from the flue temperature probe (an NTC thermistor sensor mounted in the flue outlet). This sensor continuously monitors the temperature of combustion gases leaving the boiler. It works by changing its electrical resistance in response to temperature — the PCB reads this resistance to keep track of flue conditions. When the signal falls outside expected parameters, the boiler interprets this as a safety risk and locks out to prevent damage or danger. You will typically lose both heating and hot water until the fault is cleared.
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 flue temperature probe (NTC thermistor) Common
The probe itself is the most frequent culprit. Over time the thermistor can drift out of calibration, develop an internal open or short circuit, or simply fail outright. This produces a resistance reading that falls outside what the PCB expects, triggering the ER09 lockout. A straightforward probe replacement usually resolves the fault.
- Damaged or corroded wiring connections Sometimes
The wiring loom connecting the flue probe to the PCB can suffer from corrosion at the terminals, loose plug connectors, or physical damage from heat or vibration. Even a high-resistance connection can cause the PCB to misread the probe signal and log ER09.
- Flue obstruction causing abnormal flue temperatures Sometimes
A partially blocked or incorrectly sized flue terminal can trap hot combustion gases, causing genuine temperature spikes that confuse the probe or push it into fault territory. Birds' nests, debris, or a kinked flue pipe are common culprits, particularly after stormy weather.
- PCB fault Rare
If the probe and wiring check out correctly, the PCB itself may have developed a fault on the sensor input circuit — for example a damaged analogue input or a failed reference voltage. This is the least common cause and is only diagnosed after all other possibilities have been ruled out.
How to fix it
- Reset the boiler (up to twice) DIY safe
Locate the reset button on the boiler's front panel and hold it for approximately 5–10 seconds until the display changes or the boiler attempts to restart. If the ER09 code returns immediately or after a short run, do not keep resetting — repeated failed attempts will not clear an underlying fault and can mask the problem.
- Check that the flue terminal is clear of obstructions DIY safe
Go outside and look at where the flue exits the building. Make sure the plastic terminal is not blocked by debris, a bird's nest, or a build-up of ice in very cold weather. Do not reach inside the flue — simply check for obvious external blockages and clear any loose debris carefully.
- Have a Gas Safe engineer test and replace the flue temperature probe Gas Safe engineer
A qualified engineer will use a multimeter to measure the probe's resistance at a known temperature and compare it against the manufacturer's specification. If out of range, the probe is replaced — a genuine Biasi flue NTC probe (e.g. part BI1782103) costs roughly £30–£50 + VAT and is widely available for next-day delivery. This single step resolves the majority of ER09 faults.
- Have the engineer inspect the probe wiring and connectors Gas Safe engineer
If fitting a new probe does not clear the fault, the engineer should inspect the wiring loom between the probe and the PCB for signs of corrosion, heat damage, or loose terminals. Damaged sections of wiring should be repaired or replaced — this is electrical work inside the boiler and must not be attempted by a homeowner.
- Have the PCB assessed and replaced if necessary Gas Safe engineer
If both the probe and wiring are in good condition but the ER09 fault persists, the PCB sensor input circuit is likely at fault. The engineer will test the reference voltage and signal path on the board. PCB replacement is a specialist job and only worthwhile on younger boilers — on an older system, the engineer may advise considering a new boiler instead.
- Call a Gas Safe registered engineer if you have not already done so Gas Safe engineer
If the steps above have not resolved the fault, or if you are unsure at any point, contact a Gas Safe registered engineer. You can find one at the Gas Safe Register website (gassaferegister.co.uk). Always ask to see their Gas Safe ID card before any work begins.
Parts you may need
- Biasi flue temperature NTC probe (e.g. BI1782103) · from £45
- Probe wiring harness / connector loom · from £25
- Biasi PCB (model-specific) · from £180
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–£300, depending on the underlying cause.
Frequently asked questions
Can I fix the Biasi ER09 fault myself?
The only homeowner-safe actions are resetting the boiler (no more than twice) and checking the external flue terminal for obvious blockages. All internal work — including testing or replacing the flue probe, inspecting wiring, or touching the PCB — must be carried out by a Gas Safe registered engineer. Working on the internals of a gas appliance without the correct registration is illegal and dangerous.
How much does it cost to fix an ER09 fault in the UK?
In most cases the repair involves replacing the flue temperature probe, which costs £30–£50 for the part. Add a Gas Safe engineer's call-out and labour and the typical total is £150–£300 depending on your location and the engineer's rates. If the fault turns out to be a PCB issue, parts and labour together can reach £350–£500 or more — at that point, on an older boiler, it is worth getting a new boiler quote for comparison.
Will my Biasi warranty cover the ER09 fault?
Biasi boilers are generally sold with a 5–7 year warranty depending on the model. If your boiler is still within the warranty period, contact Biasi or your installer before arranging an independent repair, as an unauthorised repair could void the remaining cover. Have your installation date and model details to hand when you call.
Why does my Biasi boiler keep showing ER09 after I reset it?
If the fault returns quickly after a reset, a simple reset is not going to fix the underlying problem. The most likely cause is a probe that has genuinely failed and needs replacing. It can also point to a wiring connection issue or, less commonly, a PCB fault. Repeated resetting will not resolve any of these — arrange for a Gas Safe engineer to diagnose the fault properly.