Main Eco Compact E321 / 163 fault code: domestic hot-water NTC sensor fault

A Main Eco Compact displaying E321 / 163 is reporting a domestic hot-water NTC sensor fault. Baxi’s current official error table lists the code twice for the relevant model group: once for a short-circuit condition and once for an open-circuit condition.

The display does not tell a homeowner which circuit condition is present, and it does not prove that the sensor itself must be replaced. The sensor, its connections and the boiler controls form a monitored circuit that needs professional testing.

What does Main Eco Compact E321 / 163 mean?

E321 and 163 are paired versions of the same fault code. “DHW” means domestic hot water: the water supplied to taps, showers or a connected hot-water arrangement. “NTC” is a type of temperature sensor whose electrical behaviour changes with temperature, allowing the boiler controls to interpret how hot the water is.

The official table describes two possible circuit states:

• domestic hot-water NTC sensor fault, short circuit; and
• domestic hot-water NTC sensor fault, open circuit.

In simple terms, the control is seeing an electrical reading outside the expected pattern for that temperature-sensing circuit. “Open” and “short” are technical circuit descriptions; they are not instructions to open the boiler or bridge anything.

Which Main Eco Compact boilers does it cover?

Baxi groups E321 / 163 with Main Eco Compact Combi and System boilers. The hot-water arrangement differs between installations: a combi heats tap water directly, while a system boiler can work with a separate cylinder and controls. What the household notices can therefore vary.

Give the engineer the complete model name, output and whether the home uses a hot-water cylinder. Do not assume a guide or replacement part for one version applies to the other.

What symptoms might appear?

On a combi, hot water may be unavailable, alternate between hot and cold, or stop heating as expected. The boiler may show E321 / 163 when a hot tap is opened. Central heating might behave differently because this code specifically refers to the domestic hot-water temperature circuit.

With a system boiler and cylinder, the household symptom may be less direct. Hot-water timing, cylinder temperature and any other displayed message provide useful context, but do not establish the cause on their own.

The display may remain lit while the boiler stops the relevant operation. A fault that clears temporarily can still return when the controls next check the circuit.

Safe observations to record

Without removing any covers, note:

• whether the display says E321, 163 or alternates between them;
• a clear photograph of the complete display;
• the exact boiler model and output from the outside label or paperwork;
• whether the appliance is a combi or works with a hot-water cylinder;
• whether hot water is cold, intermittent, unusually hot or simply unavailable;
• whether central heating continues to work normally;
• whether the code appears only when a hot tap or timed hot-water demand starts;
• any other code shown immediately before or after it; and
• any recent professional work on the boiler, cylinder or controls.

Do not run water for a long time purely to reproduce the fault. A short observation of normal use is enough.

Can you reset E321 / 163?

If the exact user guide and the display indicate that reset is allowed, try it once. Baxi’s published reset guidance for this control style says to press the reset control for approximately one second; holding it for longer than two seconds will not reset the boiler.

If E321 / 163 remains or returns, stop resetting and arrange repair. A reset may restart the control sequence, but it cannot repair an open or shorted sensor circuit. Repeated resets can obscure when and how the code appears.

What not to do

Do not remove the boiler casing, unplug a sensor, inspect connectors or test resistance, voltage or continuity. Do not bridge terminals, alter service settings or replace a sensor based solely on the code. These actions involve internal electrical and safety-critical parts.

Do not turn the hot-water temperature to extremes in an attempt to clear the message. Avoid copying a test intended for a different Baxi or Main model. Similar codes can have different meanings across control families.

If tap water becomes unexpectedly very hot, stop using it until the appliance has been assessed, particularly where children, older people or anyone with reduced sensitivity could be at risk of scalding.

Why professional checks are needed

An engineer can confirm the exact model, read the stored fault history and test the temperature-sensing circuit using the manufacturer’s information. The checks can distinguish the sensor element from a connection, wiring or control issue without assuming the first visible part is responsible.

The boiler casing should remain closed because the relevant components are inside the appliance or associated with its professional installation. Correct testing also confirms that hot-water temperature control is dependable before normal use resumes.

When should you book a repair?

Arrange repair if the code remains after one permitted reset, returns later or leaves the home without reliable hot water. Book promptly if the water temperature is unpredictable, especially if it becomes hotter than expected.

If the boiler also fails to start for heating, our guide to a boiler not firing up lists safe information worth recording. Make E321 / 163 and the hot-water symptoms clear in the booking notes.

Gas and carbon-monoxide safety

E321 / 163 is a temperature-sensor circuit code, but the boiler still burns gas. If you smell gas, suspect a leak or a carbon-monoxide alarm sounds, leave the property, avoid electrical switches and call the National Gas Emergency Service on 0800 111 999 from a safe place. Seek urgent medical help for possible carbon-monoxide symptoms.

Quick Answers

What does Main Eco Compact E321 / 163 mean?

It means the boiler has detected a fault in the domestic hot-water NTC temperature-sensor circuit.

What do open circuit and short circuit mean?

They are two different electrical conditions that the boiler can detect in the same monitored circuit. Both require professional testing.

Does E321 / 163 prove the sensor has failed?

No. The sensor, its connections, wiring and controls need checking before a cause is confirmed.

Can central heating still work?

It may, because the code specifically concerns the domestic hot-water temperature circuit. The exact behaviour depends on the model and installation.

Can I reset E321 / 163?

If the display and exact user guide allow it, try one reset. If the code remains or returns, stop resetting and book a repair.