A Main boiler showing E20 has detected a fault in the central-heating temperature-sensor circuit on models that use this code. It normally needs professional diagnosis rather than repeated resets.
The exact code format and boiler model matter. On documented Main Eco Compact and Eco Elite models, E20 means a central-heating NTC fault. An NTC is a temperature sensor whose electrical resistance changes with temperature, allowing the boiler controls to monitor the water safely.
What does Main boiler fault code E20 mean?
Main’s documentation lists E20 as a “central heating NTC fault”. Baxi’s current official error-code resource, which covers Main Eco Compact combi and system boilers in the relevant group, describes E20 more specifically as a central-heating NTC sensor short circuit or open circuit.
In plain English, the boiler is not receiving a believable signal from the heating temperature-sensor circuit. That does not prove the sensor itself is the only possible failed part. The sensor, its connections, associated wiring or the way the boiler electronics receive the signal all require proper testing before any part is condemned.
Check the full code, including punctuation
Do not treat every code containing “20” as the same fault. The simple legacy-style code E20 is not automatically equivalent to newer dotted codes such as E.00.20 used on some other Baxi-family controls. Manufacturer documentation assigns different meanings to different code formats and product generations.
Photograph the display and the boiler’s model label without removing the casing. If the display alternates between parts of a three-digit code, wait long enough to record the whole sequence. Giving the engineer the exact model and complete code prevents an article or parts list for the wrong boiler being applied to yours.
What might you notice alongside E20?
The boiler may stop responding to a heating demand, lock out, or leave the radiators cold. On a combi model, hot-water behaviour can vary because the controls may prevent operation when a temperature signal cannot be trusted. The display behaviour is model-specific, so the code is more reliable evidence than guessing from one symptom.
You might also notice that the boiler had behaved intermittently before the code became permanent. Intermittency does not make the fault safe to ignore. A connection problem can appear and disappear with heat or vibration, while a sensor can produce an implausible reading only at certain temperatures.
Is E20 a low-pressure code?
No. In the Main manuals that list E20, low system pressure has a different code. For example, Main Eco Compact documentation identifies E118 as primary-system water pressure below 0.5 bar, while some earlier Main models use E119 for a pressure-switch condition.
Do not add water simply because the boiler has stopped. If the gauge is also outside the normal range, follow only the model’s own user instructions or tell the engineer what it shows. Repressurising an already correctly filled system can create a second problem without clearing E20.
Can I reset a Main boiler showing E20?
The Main Eco Compact service manual lists a defined group of codes that can be cleared with its reset button; E20 is not in that reset list. Older Main user instructions likewise tell the user to note E20 and contact the installer or service engineer.
That is the safest approach: record the code and arrange diagnosis. Do not keep cycling the power, repeatedly press reset, remove the case or disturb sensor wiring. A reset cannot repair an open or short-circuit sensor circuit, and repeated attempts can erase useful evidence about when the fault occurs.
What can a homeowner check safely?
- Confirm that the display really says E20 and record whether it is constant or intermittent.
- Write down the exact Main model from the accessible label or your paperwork.
- Note whether heating, hot water or both stopped, and what the boiler was doing immediately beforehand.
- Check only normal external controls, such as whether the programmer and room thermostat are calling for heat.
- Leave the boiler casing closed and arrange a suitably qualified Gas Safe registered engineer.
These observations can shorten diagnosis without turning the homeowner into the fault finder. Electrical resistance tests, continuity checks and any work inside the boiler belong to the attending engineer.
What will the engineer need to establish?
The engineer must confirm the model-specific code definition, inspect the relevant temperature-sensor circuit and test it against the manufacturer’s service information. That process distinguishes a failed sensor from a connection, wiring or control problem.
A responsible diagnosis comes before parts replacement. E20 names the circuit the boiler is unhappy with; it does not by itself identify which component must be bought.
Is an E20 fault dangerous?
The code is a protective response to an unreliable temperature reading. Treat the boiler as unavailable until it has been assessed, but do not assume that the display alone means gas is escaping.
If you smell gas, suspect carbon monoxide, feel unwell with possible carbon-monoxide symptoms, or a carbon-monoxide alarm sounds, leave the property and follow the emergency instructions from the National Gas Emergency Service. Do not wait for an ordinary repair appointment.
Does E20 apply to every Main boiler?
No. Main has produced several control platforms, and not every model uses the same code set. This guide applies only where the correct manufacturer manual for the exact boiler lists the simple code E20 as the central-heating NTC fault.
If the boiler shows a dotted code, a symbol, a flashing-light pattern or a different number, use the documentation for that model. Similar-looking Main boilers can have different controls and different fault meanings.
Frequently asked questions
Will E20 clear when the boiler cools down?
It may appear intermittent, but cooling does not prove the circuit is healthy. Record what happens and have a recurring E20 investigated.
Can replacing the temperature sensor always fix it?
No. The code refers to the sensor circuit. The engineer must test the sensor and associated connections before deciding which repair is justified.
Can I keep using an immersion heater?
If your property has a separate hot-water cylinder and immersion heater, that appliance is separate from the boiler. Use it only in accordance with its own controls and instructions.
What information should I send when booking?
A photograph of the complete display, the model name, whether heating and hot water are affected, and when the code first appeared are useful. Never remove the casing to obtain a photograph.
Main boiler still showing E20?
Have the model-specific temperature-sensor circuit diagnosed safely instead of replacing parts on the strength of the code alone.