Biasi ER19 Fault Code: Causes, Fixes & Repair Costs
What does the Biasi ER19 fault code mean?
The ER19 code (sometimes shown as ER + 19 on the boiler display) indicates that the boiler's control board is not receiving a reliable signal from the central heating (CH) flow temperature probe — also known as an NTC thermistor. This sensor continuously monitors the water temperature circulating through the heating circuit and feeds that data back to the PCB. When the signal falls outside acceptable parameters — whether due to a broken sensor, wiring problem, or heavy limescale coating — the boiler locks out as a safety measure rather than risk overheating or running blind. Until the probe is sending accurate readings again, the boiler will refuse to fire.
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 NTC thermistor (temperature probe) Common
The NTC thermistor is an electrical component that changes resistance as temperature changes. Over time — typically after 8–12 years of continuous heating cycles — the sensor element degrades or fails outright, sending readings that are either wildly inaccurate or flatline completely. This is by far the most common trigger for ER19 and is usually confirmed by a Gas Safe engineer testing resistance values with a multimeter.
- Loose or corroded wiring connections Common
The small connector plug linking the thermistor to the PCB harness can work loose due to vibration or corrode in the damp environment inside the boiler casing. Even a high-resistance connection caused by mild corrosion is enough to corrupt the signal and throw the lockout code. The engineer will inspect the plug, pins, and wiring run back to the board.
- Limescale coating on the probe Sometimes
In hard-water areas (much of South East England and the Midlands), limescale can build up on the probe body, effectively insulating it from the water temperature it is supposed to measure. The sensor then reads lower temperatures than reality, causing erratic control and eventually a lockout. A descale or system flush may be required alongside the probe replacement.
- PCB (printed circuit board) fault Rare
If the probe and all its wiring check out fine with test equipment, attention turns to the PCB itself. A failing control board can misinterpret a perfectly good sensor signal or lose the input channel entirely, manifesting as an ER19 code. PCB faults are less common but more expensive — this diagnosis is normally reached after the sensor has been ruled out.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button on the boiler fascia for around 3–5 seconds (refer to your model's handbook if the button location is unclear). The boiler will attempt to restart. If it fires up and runs normally, monitor it over the next few hours. If ER19 returns, do not keep resetting repeatedly — doing so can mask the underlying fault and put additional strain on components.
- Check your system pressure DIY safe
Look at the pressure gauge on the boiler — it should sit between 1 and 1.5 bar when the system is cold. If it is below 1 bar, use the filling loop to top it up to around 1.2 bar. A significantly low or fluctuating pressure can sometimes contribute to sensor-related error codes, though it is unlikely to be the sole cause of ER19.
- Do not attempt to access or test the probe yourself Gas Safe engineer
The NTC thermistor and its wiring are internal components connected to the boiler's electrical system. Testing or replacing the sensor involves working inside the boiler casing and handling electrical connections on a gas appliance — this must only be done by a Gas Safe registered engineer. Do not remove the boiler casing or disconnect any wiring.
- Have a Gas Safe engineer test the thermistor and wiring Gas Safe engineer
The engineer will use a multimeter to measure the probe's resistance at a known temperature and compare it against Biasi's specification. They will also check the wiring loom and connector plug for corrosion, damage, or poor contact. In most ER19 cases, a replacement thermistor (a relatively inexpensive part) resolves the fault quickly.
- If the probe is fine, request PCB investigation Gas Safe engineer
Should the thermistor and wiring test as healthy, the engineer will investigate the PCB. A faulty control board can misread a good sensor signal. PCB replacement is a more involved and costly repair — get a clear written quote before authorising this work, and weigh it against the age and overall condition of the boiler.
Parts you may need
- NTC thermistor / CH temperature probe · from £20
- Wiring harness connector / probe plug · from £12
- PCB (printed circuit board) · 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 £120–£280, depending on the underlying cause.
Frequently asked questions
Can I fix Biasi ER19 myself by just resetting the boiler?
A single reset is a perfectly reasonable first step and costs nothing. However, ER19 points to a component fault — usually a failed temperature probe — that a reset will not cure. If the code comes back after one reset, the underlying problem needs a Gas Safe engineer to diagnose and repair. Repeatedly resetting a locked-out boiler is not recommended, as it can put additional stress on components and may mask a developing fault.
How much does it cost to fix Biasi ER19 in the UK?
For the most common repair — replacing the NTC thermistor — expect to pay roughly £120–£200 including parts and labour. If the wiring harness also needs attention, you might be closer to £180–£280 all-in. A PCB replacement (a less common outcome) typically costs £250–£400 or more and should prompt a conversation about whether repairing an older boiler still makes financial sense. As a rough rule of thumb, if the repair quote exceeds around £500, a new boiler is often the more sensible long-term investment.
Is my Biasi boiler still under warranty?
Biasi boilers generally carry a manufacturer's warranty of between 5 and 7 years depending on the specific model and when it was registered. If your boiler is within that window, contact Biasi directly before arranging an independent engineer — a warranty repair could save you the full cost. Topping up the pressure via the filling loop and resetting the boiler are both considered normal user actions and do not affect the warranty.
Will limescale cause ER19 on my Biasi boiler?
It can, particularly if you live in a hard-water area such as London, the Home Counties, or the East Midlands. A thick coating of limescale on the probe body insulates it from the water around it, meaning the sensor reads inaccurately. If your engineer finds the probe is heavily scaled rather than failed electrically, they may recommend a descale of the heat exchanger or a full system powerflush alongside fitting the new sensor, to prevent the same issue recurring.