Main boiler fault code E321, which may also be shown as 163 on some displays, is listed by Baxi’s current official error-code reference as a domestic-hot-water NTC sensor fault for the appliance group that includes Main Eco Compact Combi and System boilers. The manufacturer lists both short-circuit and open-circuit versions of the code.
In plain English, the boiler is receiving a temperature-sensor signal that is outside the range it expects. That does not prove the sensor itself needs replacing. The sensor, its connections, associated wiring and the controls form one circuit, so the correct repair depends on proper testing.
What does Main E321 or 163 mean?
NTC stands for negative temperature coefficient. An NTC sensor changes its electrical resistance as temperature changes, allowing the boiler controls to monitor hot-water temperature. If the signal appears electrically open or shorted, the boiler cannot rely on that reading and may stop hot-water operation or lock out.
Baxi’s published table uses the wording “Domestic Hot Water NTC sensor fault” and gives separate entries for short circuit and open circuit. Those are technical descriptions of the signal condition, not instructions for a homeowner to test the component.
Which Main boilers can show E321?
The current Baxi error-code group includes Main Eco Compact Combi and Main Eco Compact System models. Main has produced other ranges with different control systems, so E321 should not be assumed to mean the same thing on every boiler carrying the Main name.
Check the complete model name on the front, user paperwork or Benchmark record. Photograph the whole display because the number, any accompanying symbol and the way the code flashes can help identify the correct model literature.
Why can a System boiler show a hot-water sensor code?
A combi boiler heats domestic water as a tap is opened. A system boiler usually works with a separate hot-water cylinder and its controls. The way the temperature signal is used therefore depends on the installation and exact appliance. The official model information should be followed rather than assuming the plumbing arrangement from the code alone.
If you have a cylinder, note whether stored hot water is cold, unusually hot or intermittent. Do not remove cylinder thermostats or covers and do not alter wiring centres or motorised valves.
What symptoms may appear with E321?
- No hot water when a tap is opened.
- Hot water that starts and then turns cold.
- Water temperature that is higher or lower than the selected setting.
- A boiler that locks out when hot water is requested.
- Central heating that still operates while hot water does not.
Not every home will experience all of these symptoms. The displayed code and model details are more useful than trying to identify a particular component from the symptoms alone.
Can I reset Main E321?
The Main Eco Compact service literature describes E321 as a possible component fault and does not include it among the listed codes that can normally be cleared by the user reset procedure. Follow the user guide for your exact appliance rather than repeatedly pressing reset.
If the code briefly disappears and returns, the underlying signal problem remains. Repeated resets can also remove useful information about when the fault occurs.
Safe information to collect
- Take a clear photograph of E321, 163 and any symbols.
- Record the exact Main boiler model.
- Note whether heating works normally.
- Check whether the problem affects every hot tap or only one outlet.
- Record whether the water is cold, intermittent or unexpectedly hot.
- Note whether the fault followed a power interruption or recent work.
You can use the normal controls and read the pressure display without opening the casing. Do not test the sensor, measure resistance, disconnect plugs or bridge any terminals.
Is E321 dangerous?
The code is a protective response to an unreliable temperature signal. Keep the boiler casing closed. If water is becoming unexpectedly hot, stop using the affected outlet until the fault is repaired, especially where children, older people or anyone with reduced sensitivity may be at risk of scalding.
Switch the appliance off and seek prompt help if water is leaking onto electrical equipment. If you smell gas or suspect carbon monoxide, move to fresh air and call the National Gas Emergency Service on 0800 111 999. Seek medical help if anyone feels unwell.
What will the engineer check?
The engineer will confirm the boiler model, review the displayed and stored fault information and observe what happens when hot water is requested. The relevant temperature-sensing circuit, connections and boiler controls can then be checked safely.
The result may point to a sensor, connection, wiring issue or control fault. Parts should be selected from the evidence and exact appliance details, not from the code number alone.
How is E321 different from a general no-hot-water problem?
E321 provides a specific temperature-sensor circuit message. A home can lose hot water for many other reasons, including controls, water flow, system configuration or another boiler fault. If there is no E321 or 163 display, start with our guide to no hot water while the heating still works.
What should I send when booking?
Send the display photograph, full model name and a short description of the hot-water behaviour. Include whether heating still works, whether you have a hot-water cylinder and whether any electrical or plumbing work happened shortly before the code appeared.
Get your Main boiler’s hot-water fault fixed.
Book a fixed-price boiler repair so the E321 fault can be checked safely and the correct repair carried out.
Main E321 FAQs
Are E321 and 163 the same fault?
They are paired in Baxi’s current table for the applicable Main Eco Compact group and relate to the domestic-hot-water NTC sensor circuit.
Does E321 mean the sensor has definitely failed?
No. The complete signal circuit needs checking before a part is chosen.
Can I test the NTC sensor myself?
No. That requires access inside the boiler and electrical testing. Keep the casing closed.
Why does the heating still work?
The problem may affect the hot-water temperature signal while the heating circuit can still operate. Normal heating does not mean the fault has repaired itself.
Can I keep using hot water?
If the temperature is unpredictable or unusually hot, stop using the affected outlet until the boiler is repaired.