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Main boiler error codes

13 codes that Main (Baxi Heating UK) publishes for its boilers, each with what it means, the likely cause, what you can safely check and when to call a Gas Safe registered engineer. Tap your code:

Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone. Anything else is for a Gas Safe registered engineer. If you smell gas, call the National Gas Emergency Service on 0800 111 999. Codes that need one are marked Gas Safe engineer.

Read this first

Main is a Baxi Heating UK brand, and its user guides are published on mainheating.co.uk. This page follows the user guide for the Main Eco Compact Combi (25 and 30 kW), which shows a plain number with a sign: a spanner means the fault cannot be reset by you, and the reset sign R means you may clear it by pressing Reset for about a second. Its table lists 19 codes. Thirteen are explained below; six carry exactly the fault names of codes already explained on the Baxi boiler page and are linked there. The guide adds that 103, 104, 107 and “Bat” mean you should contact your installer, annual service provider or Baxi Customer Support — it gives no meaning for them, so neither do we.

118 Water pressure too low

What it means
The primary (heating) water pressure is too low. The guide says the boiler will not work below 0.5 bar and that the system must be topped up below 0.7 bar.
Likely cause
Water lost from the heating system.
What you can safely check

Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.

  • Read the pressure gauge or the display: normal operating pressure is 1 to 2 bar.
  • Repressurise with the filling loop: connect the filling pipe between the two taps, open one fully, then open the second slowly while watching the gauge. Turn both off at 1 to 1.5 bar, then disconnect the pipe. The guide says the boiler resets itself once the pressure is right.
When to call a Gas Safe registered engineer
If you have to repressurise regularly, the guide says a fault or leak is indicated: seek advice from your installer or a Gas Safe registered engineer. A pressure above 3 bar makes the safety valve operate — contact your installer.

Source: Main (Baxi Heating UK), Main Eco Compact Combi 25–30 user guide: 7.1 error codes, 7.2 fault finding, 6.2.1 re-pressurising the system Checked 2026-09-29.

09 Gas valve connection cable

What it means
The boiler has found a fault in the cable that connects its gas valve.
Likely cause
The gas valve cable or its connection inside the boiler.
What you can safely check

Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.

  • If the display shows the reset sign R beside the code, press the reset button for about 1 second (the guide says holding it for more than 2 seconds will not reset the boiler). If it shows the spanner instead, the guide says you cannot reset it.
When to call a Gas Safe registered engineer
If it does not clear, shows the spanner, or keeps coming back, contact your installer, annual service provider or Baxi Customer Support (0344 871 1545) for a Gas Safe registered engineer.

Source: Main (Baxi Heating UK), Main Eco Compact Combi 25–30 user guide: 7.1 error codes, 7.2 fault finding, 6.2.1 re-pressurising the system Checked 2026-09-29.

15 Gas valve fault

What it means
The boiler has found a fault in its gas valve.
Likely cause
The gas valve, a sealed part inside the boiler.
What you can safely check

Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.

  • If the display shows the reset sign R beside the code, press the reset button for about 1 second (the guide says holding it for more than 2 seconds will not reset the boiler). If it shows the spanner instead, the guide says you cannot reset it.
When to call a Gas Safe registered engineer
If it does not clear, shows the spanner, or keeps coming back, contact your installer, annual service provider or Baxi Customer Support (0344 871 1545) for a Gas Safe registered engineer.

Source: Main (Baxi Heating UK), Main Eco Compact Combi 25–30 user guide: 7.1 error codes, 7.2 fault finding, 6.2.1 re-pressurising the system Checked 2026-09-29.

28 Flue sensor fault

What it means
Flue NTC fault: the temperature sensor in the flue has failed or gives no reading.
Likely cause
The flue temperature sensor (NTC) or its wiring.
What you can safely check

Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.

  • If the display shows the reset sign R beside the code, press the reset button for about 1 second (the guide says holding it for more than 2 seconds will not reset the boiler). If it shows the spanner instead, the guide says you cannot reset it.
When to call a Gas Safe registered engineer
If it does not clear, shows the spanner, or keeps coming back, contact your installer, annual service provider or Baxi Customer Support (0344 871 1545) for a Gas Safe registered engineer.

Source: Main (Baxi Heating UK), Main Eco Compact Combi 25–30 user guide: 7.1 error codes, 7.2 fault finding, 6.2.1 re-pressurising the system Checked 2026-09-29.

40 Heating return sensor fault

What it means
Central heating return NTC fault: the sensor on the water coming back from the radiators has failed or gives no reading.
Likely cause
The return temperature sensor (NTC) or its wiring.
What you can safely check

Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.

  • If the display shows the reset sign R beside the code, press the reset button for about 1 second (the guide says holding it for more than 2 seconds will not reset the boiler). If it shows the spanner instead, the guide says you cannot reset it.
When to call a Gas Safe registered engineer
If it does not clear, shows the spanner, or keeps coming back, contact your installer, annual service provider or Baxi Customer Support (0344 871 1545) for a Gas Safe registered engineer.

Source: Main (Baxi Heating UK), Main Eco Compact Combi 25–30 user guide: 7.1 error codes, 7.2 fault finding, 6.2.1 re-pressurising the system Checked 2026-09-29.

83 Communication error

What it means
The boiler reports a communication error; the guide does not say between which parts.
Likely cause
Inside the boiler’s controls or wiring to them.
What you can safely check

Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.

  • If the display shows the reset sign R beside the code, press the reset button for about 1 second (the guide says holding it for more than 2 seconds will not reset the boiler). If it shows the spanner instead, the guide says you cannot reset it.
When to call a Gas Safe registered engineer
If it does not clear, shows the spanner, or keeps coming back, contact your installer, annual service provider or Baxi Customer Support (0344 871 1545) for a Gas Safe registered engineer.

Source: Main (Baxi Heating UK), Main Eco Compact Combi 25–30 user guide: 7.1 error codes, 7.2 fault finding, 6.2.1 re-pressurising the system Checked 2026-09-29.

109 Pre-circulation fault

What it means
A fault found while the pump was circulating water before the burner lit.
Likely cause
The guide names the fault only; it concerns the water moving through the boiler.
What you can safely check

Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.

  • Read the pressure gauge; below 0.5 bar the boiler will not work (see 118).
  • If the display shows the reset sign R beside the code, press the reset button for about 1 second (the guide says holding it for more than 2 seconds will not reset the boiler). If it shows the spanner instead, the guide says you cannot reset it.
When to call a Gas Safe registered engineer
If it does not clear, shows the spanner, or keeps coming back, contact your installer, annual service provider or Baxi Customer Support (0344 871 1545) for a Gas Safe registered engineer.

Source: Main (Baxi Heating UK), Main Eco Compact Combi 25–30 user guide: 7.1 error codes, 7.2 fault finding, 6.2.1 re-pressurising the system Checked 2026-09-29.

128 Flame failure

What it means
The boiler lost its flame, or did not detect one.
Likely cause
The guide names the fault only; its fault-finding table sends you first to the gas supply.
What you can safely check

Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.

  • Check that the gas isolation cock is properly open, and if the property has a prepayment meter, that it has credit.
  • If the display shows the reset sign R beside the code, press the reset button for about 1 second (the guide says holding it for more than 2 seconds will not reset the boiler). If it shows the spanner instead, the guide says you cannot reset it.
When to call a Gas Safe registered engineer
If it does not clear, shows the spanner, or keeps coming back, contact your installer, annual service provider or Baxi Customer Support (0344 871 1545) for a Gas Safe registered engineer.

Source: Main (Baxi Heating UK), Main Eco Compact Combi 25–30 user guide: 7.1 error codes, 7.2 fault finding, 6.2.1 re-pressurising the system Checked 2026-09-29.

135 Gas supply interrupted (internal error)

What it means
Interruption of the gas supply, reported as an internal error of the boiler’s control.
Likely cause
The gas supply to the burner stopped; the guide does not say more.
What you can safely check

Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.

  • Check that the gas isolation cock is properly open, and if the property has a prepayment meter, that it has credit.
  • If the display shows the reset sign R beside the code, press the reset button for about 1 second (the guide says holding it for more than 2 seconds will not reset the boiler). If it shows the spanner instead, the guide says you cannot reset it.
When to call a Gas Safe registered engineer
If it does not clear, shows the spanner, or keeps coming back, contact your installer, annual service provider or Baxi Customer Support (0344 871 1545) for a Gas Safe registered engineer.

Source: Main (Baxi Heating UK), Main Eco Compact Combi 25–30 user guide: 7.1 error codes, 7.2 fault finding, 6.2.1 re-pressurising the system Checked 2026-09-29.

154 Flow and return sensor test failed

What it means
The boiler’s check of its flow and return temperature sensors did not pass.
Likely cause
The flow or return sensor (NTC), or the water flow between them.
What you can safely check

Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.

  • If the display shows the reset sign R beside the code, press the reset button for about 1 second (the guide says holding it for more than 2 seconds will not reset the boiler). If it shows the spanner instead, the guide says you cannot reset it.
When to call a Gas Safe registered engineer
If it does not clear, shows the spanner, or keeps coming back, contact your installer, annual service provider or Baxi Customer Support (0344 871 1545) for a Gas Safe registered engineer.

Source: Main (Baxi Heating UK), Main Eco Compact Combi 25–30 user guide: 7.1 error codes, 7.2 fault finding, 6.2.1 re-pressurising the system Checked 2026-09-29.

270 Circulation fault (dry fire)

What it means
The boiler found that it was heating without enough water circulating through it — a dry-fire condition.
Likely cause
Too little water moving through the heat exchanger.
What you can safely check

Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.

  • Read the pressure gauge; below 0.5 bar the boiler will not work (see 118).
  • If the display shows the reset sign R beside the code, press the reset button for about 1 second (the guide says holding it for more than 2 seconds will not reset the boiler). If it shows the spanner instead, the guide says you cannot reset it.
When to call a Gas Safe registered engineer
If it does not clear, shows the spanner, or keeps coming back, contact your installer, annual service provider or Baxi Customer Support (0344 871 1545) for a Gas Safe registered engineer.

Source: Main (Baxi Heating UK), Main Eco Compact Combi 25–30 user guide: 7.1 error codes, 7.2 fault finding, 6.2.1 re-pressurising the system Checked 2026-09-29.

321 Hot water sensor fault

What it means
Hot water NTC fault: the temperature sensor on the hot-water side has failed or gives no reading.
Likely cause
The hot water temperature sensor (NTC) or its wiring.
What you can safely check

Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.

  • If the display shows the reset sign R beside the code, press the reset button for about 1 second (the guide says holding it for more than 2 seconds will not reset the boiler). If it shows the spanner instead, the guide says you cannot reset it.
When to call a Gas Safe registered engineer
If it does not clear, shows the spanner, or keeps coming back, contact your installer, annual service provider or Baxi Customer Support (0344 871 1545) for a Gas Safe registered engineer.

Source: Main (Baxi Heating UK), Main Eco Compact Combi 25–30 user guide: 7.1 error codes, 7.2 fault finding, 6.2.1 re-pressurising the system Checked 2026-09-29.

384 False flame

What it means
The boiler detected a flame when there should be none.
Likely cause
The guide names the fault only; the flame detection is a sealed part.
What you can safely check

Gas boiler: do only the checks its user manual gives you — read the pressure gauge, top up through the filling loop, press the reset button, thaw a frozen condensate pipe at ground level. Keep the casing on and leave sealed parts alone.

  • If the display shows the reset sign R beside the code, press the reset button for about 1 second (the guide says holding it for more than 2 seconds will not reset the boiler). If it shows the spanner instead, the guide says you cannot reset it.
When to call a Gas Safe registered engineer
If it does not clear, shows the spanner, or keeps coming back, contact your installer, annual service provider or Baxi Customer Support (0344 871 1545) for a Gas Safe registered engineer.

Source: Main (Baxi Heating UK), Main Eco Compact Combi 25–30 user guide: 7.1 error codes, 7.2 fault finding, 6.2.1 re-pressurising the system Checked 2026-09-29.

Codes explained on the Baxi boiler page

Six of the Eco Compact’s codes carry the same fault name as a code in Baxi’s table, so they are explained once, on the Baxi page. The Eco Compact shows them without the E (110, not E110) and clears an R fault with a one-second press of Reset rather than a selector switch; a spanner beside the code still means it is not yours to reset. Baxi boiler error codes.

  • 20 Central heating NTC fault — the same fault name as Baxi E20
  • 110 Safety thermostat operated — the same fault name as Baxi E110
  • 125 Circulation fault (primary) — the same fault name as Baxi E125
  • 130 Flue NTC operated — the same fault name as Baxi E130
  • 133 Interruption of gas supply or flame failure — the same fault name as Baxi E133
  • 160 Fan or fan wiring fault — the same fault name as Baxi E160

Source: Main (Baxi Heating UK), Main Eco Compact Combi 25–30 user guide: 7.1 error codes, 7.2 fault finding, 6.2.1 re-pressurising the system Checked 2026-09-29.

A code that is not listed here

The older Main Eco Elite Combi shows E-codes such as E119 and E133 instead; its own user guide on mainheating.co.uk explains them. For a code that is not in the Eco Compact’s table, the guide says to contact Baxi Customer Support on 0344 871 1545.

We list only codes the source below states. If your code is not here, it may belong to a model the source does not cover: your model’s manual is the authority.

Related pages

Source

Every code on this page comes from this page, published by Main (Baxi Heating UK) and read on the date shown. Meanings are written in our own words; nothing is taken from other makes, other models or third-party code sites.