Biasi E164 Fault Code: Causes, Fixes & Repair Costs
What does the Biasi E164 fault code mean?
The E164 fault code on Biasi boilers — sometimes shown as A07 on certain models — indicates a problem with the domestic hot water (DHW) temperature sensor. Specifically, the NTC (Negative Temperature Coefficient) thermistor that monitors hot water outlet temperature has either failed, drifted out of specification, or is sending readings that suggest the hot water temperature has climbed to an unsafe level. When the boiler's control board detects this condition, it shuts the boiler down as a safety precaution, placing it into lockout. Note that E164 also appears on Baxi boilers (which share the same parent group), where it can relate to a hydraulic pressure switch rather than a DHW sensor — always confirm you are looking at a Biasi-specific manual for your exact model.
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 DHW NTC thermistor Common
The NTC thermistor is the most common culprit. Over time the resistive element inside the sensor degrades, causing it to report temperatures that bear no relation to the actual water temperature. A Gas Safe engineer can confirm failure quickly by measuring the sensor's resistance with a multimeter and comparing it to the manufacturer's temperature-resistance table.
- Limescale build-up on the secondary heat exchanger Common
In hard-water areas, limescale accumulates on the plate heat exchanger that heats domestic hot water. This acts as an insulating layer, causing localised hot spots and genuinely elevated water temperatures that trigger E164 even when the sensor itself is still working correctly. A powerflush or chemical descale of the secondary heat exchanger is needed.
- Wiring fault between sensor and PCB Sometimes
Loose terminal connections, corroded connector pins, or chafed cable insulation on the wiring loom running from the DHW sensor to the control board can produce intermittent or erroneous resistance readings — mimicking a failed sensor. This fault often appears and clears unpredictably and may be overlooked unless the engineer inspects the loom carefully.
- Failed PCB causing incorrect sensor signal interpretation Rare
If the control board itself develops a fault on the analogue input channel that reads the DHW sensor, it may misinterpret a perfectly good sensor as faulty. This is less common but should be considered if a new sensor and fresh wiring do not clear the code.
How to fix it
- Reset the boiler once or twice DIY safe
Locate the reset button on the boiler's front panel — on most Biasi models it is clearly labelled. Press and hold it for approximately 3–5 seconds until the display changes and the boiler attempts to restart. Allow a full heating or hot water cycle to complete before deciding whether the fault has cleared. Do not reset more than two or three times in total; repeated resets without a lasting fix suggest a genuine component fault that needs professional attention.
- Check your mains gas supply DIY safe
Confirm that other gas appliances in your home — such as a gas hob or gas fire — are working normally. If they are not, contact your gas supplier rather than an engineer, as the issue may be with the incoming supply rather than the boiler.
- Check boiler system pressure DIY safe
Look at the pressure gauge on the boiler front panel. It should read between 1.0 and 1.5 bar when cold. If it is below 0.8 bar, top up via the filling loop following the instructions in your Biasi user manual. Low pressure alone will not cause E164, but it is worth ruling out before calling an engineer.
- Do not attempt to access or test the DHW NTC sensor yourself Gas Safe engineer
The sensor is located inside the boiler casing and is connected to live electrical components and the hot water circuit. Testing or replacing it requires isolating the boiler electrically, draining part of the system, and using a multimeter to verify resistance values. This must only be carried out by a Gas Safe registered engineer.
- Have a Gas Safe engineer inspect and test the DHW sensor wiring loom Gas Safe engineer
An engineer should check the connector block and wiring running from the sensor to the PCB for signs of corrosion, loose pins, or heat damage. A wiring repair is often significantly cheaper than a full component replacement and should be checked before parts are ordered.
- Have the DHW NTC thermistor tested and replaced if faulty Gas Safe engineer
Using a multimeter in resistance mode, the engineer will compare the sensor's measured resistance at a known water temperature against Biasi's specification table. If the readings are outside tolerance, the sensor should be replaced. Biasi NTC sensors for the Activ A, Advance, and Inovia ranges are widely stocked at parts suppliers such as Parts Center and HRPC, and are usually available on next-day delivery.
- Arrange a powerflush or chemical descale of the secondary heat exchanger if limescale is suspected Gas Safe engineer
If the boiler is in a hard-water area and the sensor tests within specification, limescale on the plate heat exchanger is the likely cause. An engineer can flush the heat exchanger with an approved descaling chemical. In severe cases the plate heat exchanger may need replacement rather than cleaning.
- Call a Gas Safe registered engineer if the fault persists Gas Safe engineer
If resetting the boiler does not clear E164 and you have not already booked an engineer, do so now. Operating a boiler in repeated lockout without diagnosing the root cause can mask safety-relevant conditions. Ensure any engineer you hire holds a valid Gas Safe registration — you can verify this free of charge at GasSafeRegister.co.uk.
Parts you may need
- Biasi DHW NTC thermistor (temperature sensor) · from £25
- Sensor wiring loom / connector harness · from £30
- Biasi plate heat exchanger (secondary) · from £95
- Biasi 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 £140–£320, depending on the underlying cause.
Frequently asked questions
Is E164 the same as A07 on Biasi boilers?
Yes — on some Biasi models, particularly older displays, the same underlying fault is shown as A07 rather than E164. Both codes point to a problem with the domestic hot water NTC temperature sensor or an excessively high hot water temperature reading. The diagnosis and repair process is identical for both codes.
Can I fix Biasi E164 myself?
The only homeowner-safe steps are resetting the boiler (no more than two or three times) and checking that your gas supply and system pressure are normal. The actual fault — a faulty NTC sensor, wiring issue, or limescale-blocked heat exchanger — must be diagnosed and repaired by a Gas Safe registered engineer. Attempting to access or replace the sensor yourself is unsafe and could invalidate your warranty.
How much does it cost to fix Biasi E164 in the UK?
For most households, the repair involves replacing the DHW NTC sensor and costs roughly £140–£320 including parts and labour. If limescale is the cause, a powerflush or descale of the secondary heat exchanger may add to the bill. In the uncommon situation where the PCB is at fault rather than the sensor, control board replacement can cost £300–£500 — if a quote approaches or exceeds that level it is worth getting a new boiler quotation for comparison.
Why does Biasi E164 keep coming back after I reset it?
A fault that clears on reset but returns within hours or days almost always means the underlying component has genuinely failed rather than suffered a one-off glitch. Common culprits are an NTC sensor that is drifting in and out of range as it warms up, an intermittent wiring connection that flexes with heat, or ongoing limescale build-up causing repeated overtemperature trips. Repeated resets without a proper repair are not a long-term solution and you should book a Gas Safe engineer to investigate.
Does Biasi offer a warranty that covers E164 repairs?
Biasi boilers typically carry a manufacturer warranty of five to seven years depending on the model and whether the boiler was registered at the time of installation. If your boiler is still within its warranty period and has been serviced annually by a Gas Safe engineer as required, contact Biasi directly before booking an independent repair — the sensor replacement may be covered at no cost to you.