Biasi ER12 Fault Code: Causes, Fixes & Repair Costs
What does the Biasi ER12 fault code mean?
The ER12 (also displayed as E12 on newer Biasi models such as the Inovia and Advance Plus) tells you that the boiler's control board has detected a problem with the central heating return NTC probe — the small thermistor sensor clipped onto the return pipe of the primary heat exchanger. This sensor works by changing its electrical resistance as the water temperature rises or falls; the PCB reads that resistance and uses it to manage the heating circuit safely. When the probe fails outright, drifts badly out of range, or loses its electrical connection to the board, the boiler cannot confirm return-water temperature and shuts down as a precaution. Older Biasi ranges (Riva Plus, Garda, Basicacombi) show this as ER 12; newer ranges (Rinnova, Activ A Plus, Inovia) show it as E12 — the underlying fault is identical across all models.
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 return NTC probe Common
The thermistor itself has burned out or its resistance has drifted so far from the expected curve that the PCB flags it as a fault. This is the most common cause, particularly on boilers that are several years old or have seen a lot of short-cycling.
- Loose or corroded wiring connection Common
The cable between the return NTC probe and the PCB can work loose over time, especially after any recent plumbing work on the system (such as removing radiators to decorate). Corrosion at the plug or a hairline break in the wiring loom can produce the same out-of-range signal as a dead probe.
- Air trapped in the heating circuit Sometimes
A significant air pocket circulating through the heat exchanger can create erratic temperature differentials that confuse the return probe, sometimes triggering ER12 even when the probe itself is electrically sound. Bleeding radiators after plumbing work is therefore an important first step.
- Limescale or sludge build-up in the heat exchanger Sometimes
In hard-water areas, scale on the heat exchanger surfaces can cause abnormally high temperature differentials between flow and return. This places unusual readings on the return probe and can cause the PCB to flag a probe fault even if the sensor is physically intact.
- PCB fault Rare
In a small number of cases the control board itself develops a fault in the circuit that reads the probe signal, making a healthy probe appear to have failed. An engineer would diagnose this only after confirming the probe and wiring are both good.
How to fix it
- Reset the boiler once or twice DIY safe
Locate the reset button on the boiler's front panel, press and hold it for roughly 5–10 seconds until the display changes and the boiler attempts to restart. Allow up to 10 minutes for the full ignition sequence to complete. If ER12 returns immediately or after a short run, do not keep resetting — repeated resets with an underlying electrical fault will not clear the code and could mask further issues.
- Bleed the radiators and check system pressure DIY safe
If any recent plumbing work has been carried out (e.g. radiators removed and refitted), air in the circuit is a realistic trigger. Bleed each radiator from the top floor downwards using a radiator key. Afterwards check the system pressure gauge on the boiler — it should read between 1.0 and 1.5 bar when cold. If it has dropped below 1 bar, top it up via the filling loop until it reads around 1.2 bar, then close the loop valves. Once done, attempt another reset.
- Call a Gas Safe registered engineer to test the return NTC probe Gas Safe engineer
An engineer will use a multimeter to measure the probe's resistance and compare it against Biasi's published resistance-versus-temperature curve. A reading significantly outside the expected range confirms the probe has failed. This is not a task for a homeowner — it involves disconnecting electrical components on a live boiler and interpreting specific resistance values.
- Have the engineer inspect the wiring loom and probe connector Gas Safe engineer
If the probe resistance is within tolerance, the fault likely lies in the cable or plug connecting it to the PCB. The engineer will check for corrosion, a loose push-fit connector, or a broken wire — all common after any disturbance to the boiler or pipework. Damaged wiring can sometimes be repaired; a corroded connector is usually replaced.
- Have the return NTC probe replaced if confirmed faulty Gas Safe engineer
The return NTC probe is an inexpensive part (compatible sensors are widely available from UK suppliers for a few pounds; original Biasi-branded parts are also readily stocked). Swapping it is straightforward for an engineer and typically resolves ER12 in a single visit. After fitting, the engineer should run the boiler through a full heating cycle to confirm the fault does not return.
- Check for sludge or scale if the new probe does not resolve the fault Gas Safe engineer
If ER12 persists after a confirmed probe replacement, the engineer should assess the heat exchanger for limescale or magnetite sludge. A power flush or chemical descale may be required. In hard-water areas a scale inhibitor should be added and, if not already fitted, a magnetic filter installed to protect the system going forward.
- Have the PCB tested or replaced as a last resort Gas Safe engineer
If the probe, wiring, and heat exchanger all check out, the PCB's probe-reading circuit may be at fault. PCB replacement is the most expensive outcome — costs can rise significantly (see FAQ below). On older Biasi boilers, it is worth getting a new-boiler quote alongside the PCB repair quote before committing.
Parts you may need
- Biasi return NTC temperature probe (e.g. BI1001117 or compatible KG1042501) · from £15
- Probe wiring loom / connector harness · from £25
- Biasi PCB (model-specific) · from £280
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 ER12 myself?
The only things a homeowner should attempt are a single reset (up to twice), bleeding the radiators to release trapped air, and checking that system pressure is between 1.0 and 1.5 bar. All electrical and component work — testing the NTC probe with a multimeter, inspecting the wiring loom, or replacing any parts — must be carried out by a Gas Safe registered engineer. Attempting it yourself could void your warranty and is potentially unsafe.
How much does it cost to fix ER12 on a Biasi boiler?
For most households the repair comes down to a return NTC probe replacement, which typically costs £120–£280 all-in including the engineer's call-out, diagnosis, and the part. The probe itself is cheap (original and compatible sensors are widely stocked by UK parts suppliers). If the fault turns out to be the PCB rather than the probe, costs can rise to £350–£500 or more depending on the model. For older boilers where a PCB quote comes in above £500, it is worth comparing that against the cost of a new boiler before proceeding.
Why does ER12 appear after I had radiators removed?
Removing and reconnecting radiators almost always introduces air into the heating circuit. Significant air pockets can cause erratic temperature readings across the heat exchanger, which the return NTC probe picks up as an out-of-range signal. Bleeding all radiators thoroughly after any plumbing work, checking system pressure, and resetting the boiler will often clear this type of ER12. If the fault persists, have an engineer check whether the probe connector was disturbed during the work.
Is my Biasi boiler still under warranty?
Biasi offers a 5 to 7 year warranty depending on the model. If your boiler is within that period, contact Biasi directly to arrange an inspection — a covered probe replacement could save you the full repair cost. Note that resetting the boiler and topping up system pressure are considered normal user actions and do not affect your warranty cover.