Biasi E20 Fault Code: Causes, Fixes & Repair Costs
What does the Biasi E20 fault code mean?
The E20 fault code appears on certain boilers — most commonly Baxi models — to indicate that the primary NTC thermistor (the main flow temperature sensor) is not communicating properly with the PCB. The thermistor continuously measures the water temperature in the heating circuit and feeds that data to the control board. When the PCB receives a reading that is out of range, implausible, or simply absent, it has no safe way to regulate burner output, so it shuts the boiler down and displays E20. The result is a complete loss of both central heating and hot water until the underlying cause is resolved. Important note for Biasi owners: Biasi boilers use an 'ER' prefix fault system (for example ER 01, ER 06, ER 07) rather than an E-number system. If your boiler display shows 'ER 06' or 'ER 07', those are the Biasi equivalents for a heating circuit temperature sensor fault — the diagnostic approach and likely causes are very similar to E20 and the information on this page is still relevant. If your boiler is definitely displaying 'E20', you most likely have a Baxi boiler rather than a Biasi.
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
- Faulty NTC thermistor Common
The thermistor itself is the most frequent culprit. These small resistive sensors degrade over time through repeated heat cycling, moisture ingress, or simply reaching the end of their service life. As the internal resistance drifts outside the range the PCB expects, the boiler can no longer trust the temperature reading and locks out. A multimeter resistance test — compared against the manufacturer's temperature-resistance curve — will confirm or rule this out.
- Loose or damaged wiring to the thermistor Common
The sensor connects to the PCB via a low-voltage cable and plug. Vibration over years of use can work terminals loose, and damp boiler environments can cause corrosion on connectors. Rodent damage to wiring runs is also seen occasionally in loft-mounted boilers. Any break or high-resistance joint in this circuit will mimic a failed sensor and produce E20.
- Low system pressure Sometimes
If the system pressure has dropped below around 0.8–1 bar, water circulation becomes poor. Localised hot spots around the thermistor can then produce erratic temperature readings that the PCB interprets as a sensor fault. This is not the primary cause of E20 but can trigger or worsen it, and is always worth checking first as it is the one thing a homeowner can safely address.
- Sludge or limescale restricting flow Sometimes
In older or poorly maintained systems, magnetite sludge and limescale deposits can restrict water flow around the primary sensor pocket. The thermistor then sees temperature swings that do not reflect true system behaviour, potentially triggering a lockout. A powerflush or chemical inhibitor treatment is typically required to resolve this.
- Faulty PCB Rare
If the thermistor and all its wiring test within specification, the fault may lie with the PCB itself — either the analogue input circuit that reads the thermistor signal, or a wider board failure. PCB faults are comparatively uncommon but should be considered once all other causes have been eliminated by an engineer.
How to fix it
- Reset the boiler once DIY safe
Press and hold the reset button for 3–5 seconds as described in your boiler manual. A single reset can clear a temporary glitch. If E20 returns, do not keep resetting — repeated resets on a genuine sensor fault will not fix the problem and can mask a developing issue.
- Check the system pressure gauge DIY safe
Look at the pressure gauge on the boiler fascia or pipework. It should read between 1.0 and 1.5 bar when the system is cold. If it is below 1 bar, top up via the filling loop (the braided flexible hose or inline valve usually found beneath the boiler). Open the filling valve slowly, watch the gauge rise to around 1.2 bar, then close the valve firmly. Do not overfill beyond 2 bar. If pressure is fine, or if topping up does not clear E20, move to the next step.
- Check that your gas supply is working DIY safe
Confirm that other gas appliances in your home — a gas hob, for example — are working normally. If there is no gas anywhere, contact your gas supplier rather than an engineer. If gas is available elsewhere, the supply is not the issue.
- Book a Gas Safe registered engineer to test the thermistor and wiring Gas Safe engineer
An engineer will use a multimeter to measure the thermistor's resistance at a known water temperature and compare it against the manufacturer's specification table. They will also inspect the wiring harness and plug connections for corrosion, loose terminals, or damage. In most E20 cases, a fault is found at this stage.
- Engineer to replace the NTC thermistor if required Gas Safe engineer
Thermistor replacement on most Baxi models is a relatively straightforward job for an engineer — the sensor clips or screws into the primary pipework, and a new part is inexpensive. The boiler should be re-tested for stable temperature control across a full heating and hot water cycle before the engineer leaves.
- Engineer to assess the PCB if thermistor and wiring are both sound Gas Safe engineer
If the sensor tests correctly and all connections are secure but E20 persists, the PCB input circuit may be faulty. PCB diagnosis typically involves substitution testing or specialist board evaluation. Depending on the boiler's age and the cost of a replacement board, the engineer may advise whether repair or boiler replacement is more economical.
- Consider a system flush if sludge is suspected Gas Safe engineer
If the engineer finds evidence of heavy sludge or limescale — discoloured system water, cold spots on radiators, or poor flow — a powerflush followed by addition of a quality inhibitor such as Fernox F1 or Sentinel X100 may be recommended alongside the sensor repair to prevent a recurrence.
Parts you may need
- NTC thermistor (primary flow sensor) · from £15
- Thermistor wiring harness / connector plug · from £20
- 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–£300, depending on the underlying cause.
Frequently asked questions
Can I fix an E20 fault myself without calling an engineer?
The only homeowner-safe actions are a single boiler reset and checking or topping up the system pressure. Everything beyond that — testing the thermistor, inspecting wiring, or replacing components — involves electrical work on the boiler and must be carried out by a Gas Safe registered engineer. Attempting repairs yourself could be dangerous and will almost certainly void any remaining warranty.
How much does it cost to fix an E20 fault in the UK?
For the most common cause — a faulty NTC thermistor — expect to pay roughly £120 to £300 all-in for parts and labour, depending on your region and the engineer's call-out rate. A wiring repair may fall at the lower end of that range. If the PCB needs replacing, costs can rise to £400 or beyond depending on the board and boiler model, though PCB faults are much less common. Ask your engineer for a written quote before authorising any work.
My boiler is a Biasi and I can see a code with 'ER' on the display — is this the same fault?
Biasi boilers do not use the E20 code. They use an ER (Error) prefix system. ER 06 on a Biasi typically points to a fault with the central heating flow temperature sensor — which is functionally the same issue as E20 on a Baxi. The causes and diagnostic approach are very similar: the sensor, its wiring, or the PCB will be investigated. If your display shows ER 06 or ER 07, the information on this page is a useful guide, but quote your exact Biasi model and ER code when speaking to your engineer.
Why does my boiler keep coming back with E20 even after it has been repaired?
A recurring E20 usually means the root cause was not fully resolved the first time. Common reasons include an intermittent wiring connection that was missed, a system with heavy sludge causing ongoing temperature irregularities, a pressure leak that keeps dropping the system below operating range, or — more rarely — a PCB that was misdiagnosed as healthy. Ask your engineer to confirm system pressure is holding, that fresh inhibitor has been added, and that all connections were properly secured and tested under load.