Baxi E.01-.20 Fault Code: Causes, Fixes & Repair Costs
What does the Baxi E.01-.20 fault code mean?
The Baxi E.01–E.20 range covers temperature and sensor-related faults that cause the boiler to lock out as a safety measure. The most frequently seen code in this range is E.20 (sometimes displayed as E20), which points to a problem with the central heating NTC thermistor — the sensor responsible for monitoring water temperature inside the boiler. When this sensor fails, reads implausibly high or low values, or loses its connection to the control board, the boiler cannot safely regulate heating output and shuts itself down. Earlier codes in the range (E.01 through E.19) relate to various temperature threshold and flue gas temperature conditions; in practice E.20 is by far the most common fault homeowners report. In all cases the boiler will not restart until the underlying issue is resolved.
General guidance only — not a substitute for professional advice. 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 NTC thermistor is a small temperature-sensing resistor clipped onto the heating circuit. As it ages or is affected by moisture it drifts out of calibration, sending incorrect temperature readings to the control board. This is the single most common root cause of the E.20 code and usually requires the sensor to be replaced.
- Loose or corroded wiring connections Common
The thermistor connects to the PCB via a wiring loom. Vibration, age, or damp conditions inside the boiler casing can cause connector pins to loosen or corrode, breaking the signal. The boiler interprets the loss of signal as a sensor fault and locks out.
- Low system pressure or restricted water flow Sometimes
If system pressure drops below around 0.5 bar, or if sludge or scale restricts water flow around the heat exchanger, localised overheating can occur. The thermistor detects this as an abnormally high temperature and triggers a lockout even though the sensor itself may be fine.
- Limescale or sludge build-up on the heat exchanger Sometimes
In hard-water areas, scale deposits insulate the heat exchanger walls, causing hot spots. Sludge from corroding system components can block narrow waterways. Both conditions upset temperature readings and can push the boiler into overheat lockout repeatedly.
- Faulty PCB (control board) Rare
The PCB is responsible for interpreting the resistance signal from the thermistor. A damaged or failing PCB may misread a perfectly healthy sensor and flag a spurious fault. This is the least common cause and is usually only suspected after the thermistor and wiring have been ruled out.
- Power surge or water ingress Rare
A voltage spike or condensation entering the boiler casing can damage sensor circuitry or wiring insulation, producing intermittent or permanent E.20 lockouts.
How to fix it
- Check your gas supply is on DIY safe
Make sure the gas isolation valve on the supply pipe to the boiler is fully open (handle in line with the pipe). Check that other gas appliances in your home are working normally. If you have no gas at all, contact your gas supplier before doing anything else.
- Check and top up system pressure if needed DIY safe
Look at the pressure gauge on the boiler — it should read between 1.0 and 1.5 bar when the system is cold. If it is below 1.0 bar, use the filling loop to top up to around 1.2 bar following your boiler's user guide. Do not overfill beyond 2.0 bar.
- Reset the boiler (up to twice) DIY safe
Turn the selector dial to the R (Reset) position and hold it for at least 5 seconds, then release. The display should revert to the current temperature reading. If the boiler fires up and runs normally, monitor it over the next 24 hours. If the fault returns quickly, do not keep resetting — a recurring lockout indicates an underlying fault that needs professional diagnosis.
- Check the area around the boiler is clear DIY safe
Ensure that nothing is blocking the flue terminal outside (leaves, bird nests, garden equipment) and that no furniture or stored items are obstructing the air vents on the boiler casing. Restricted airflow can contribute to elevated flue temperatures.
- Contact a Gas Safe registered engineer for diagnosis and repair Gas Safe engineer
If the fault code returns after a reset or the boiler will not reset at all, a qualified engineer is needed. They will use a multimeter to measure the thermistor's resistance across a range of temperatures and compare it against the manufacturer's specification. They will also inspect the wiring loom and connectors for damage, check system pressure and flow rate, and assess whether the PCB is interpreting sensor signals correctly. Do not attempt to test or replace the thermistor, wiring, or any internal components yourself.
Parts you may need
- NTC thermistor (central heating) · from £30
- Thermistor wiring loom / connector · from £20
- Magnetic system filter (e.g. Fernox TF1) · from £55
- PCB (printed circuit 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 £120–£320, depending on the underlying cause.
Frequently asked questions
Can I keep using my Baxi boiler while it shows E.20?
No. The E.20 code is a safety lockout — the boiler has shut itself down and will not produce heat until the fault is cleared. Attempting to force repeated resets without fixing the underlying problem risks overheating the heat exchanger and causing more expensive damage. Get it looked at promptly, especially in cold weather.
How much does it cost to fix a Baxi E.20 fault?
For the most common repair — replacing the NTC thermistor — expect to pay roughly £150–£250 including parts and labour. A loose wiring connection caught quickly may come in at £100–£150. More involved work such as a system power-flush to clear sludge typically costs £300–£400 for the flush itself. A PCB replacement is the expensive outlier, often £350–£500 or more, but this is rarely the cause of an E.20 fault and should only be recommended after all other components have been tested and eliminated.
What is the difference between E.20 and the other codes in the E.01–E.19 range?
E.20 specifically flags a fault with the central heating NTC thermistor (sensor failure or out-of-range reading). The earlier codes in the E.01–E.19 range relate to temperature threshold conditions such as the maximum flue gas temperature being reached or other thermal safety limits being exceeded. In practice, E.20 is by far the most commonly reported code from this range. If your display shows a code between E.01 and E.19, the same diagnostic approach applies — check pressure, attempt a single reset, and call a Gas Safe engineer if the fault persists.
Will an annual boiler service help prevent the E.20 fault?
Yes, significantly. During a service, a Gas Safe engineer will clean the heat exchanger, check sensor resistance readings, inspect wiring connections, and verify water flow and pressure. Catching a drifting thermistor or a loose connector early prevents unexpected lockouts. Adding a magnetic filter (such as a Fernox TF1 or Adey MagnaClean) and topping up system inhibitor annually also reduces sludge build-up, which is one of the less obvious triggers for temperature-related lockouts.