Main Combi 24 Fault codes & diagnostics

13 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.

Combi Natural Gas 24 kW 2005-2012 Discontinued
GC Number 47-474-02

Replaced by: Main Combi Eco

Download the Main Combi 24 manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. PDF

This boiler uses indicator light combinations

Note which lights are on, off, or flashing and at what speed. Match the combination to the cards below. Do not reset until you have noted the pattern. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
13 High
Versions & GC numbers

Which version do you have?

The GC number on your boiler's data badge identifies the exact appliance and production years, as recorded in the UK Product Characteristics Database.

GC number Appliance (register name) Produced
47-474-02 Combi 24 HE 2005–2012
Just want the most common Main Combi 24 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 13 documented codes

Tap any card for details

15

Your boiler has lost its connection to the fuel supply, meaning it cannot ignite to produce heat or hot water.

High DIY-safe £120-200

What to check first

  1. Locate the reset button (often marked with a flame icon or the word Reset) on the front control panel.
  2. Press and hold the button for 3 to 5 seconds until the display clears.
  3. Wait a few minutes to see if the boiler fires up and holds the flame.

Parts commonly replaced

  • Gas valve wiring harness
  • Gas valve assembly
  • PCB
Full explanation for 15
20

Your boiler has detected a general internal fault that is preventing it from starting up correctly.

High Engineer only £150-300

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness
  • Gas valve

Related codes

Full explanation for 20
28

Your boiler has detected a problem with the sensor that monitors the water temperature, meaning it cannot safely heat your home or tap water.

High Engineer only £150-250

Parts commonly replaced

  • Flow NTC sensor
  • Return NTC sensor
  • Wiring harness
Full explanation for 28
40

Your boiler has detected an internal issue with its electrical controls, meaning it has safely shut down to prevent further damage.

High Engineer only £150-300

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness

Related codes

Full explanation for 40
83

Your boiler's internal components are struggling to talk to each other or the thermostat, preventing the system from starting up.

High Engineer only £100-£350

Parts commonly replaced

  • Printed Circuit Board (PCB)
  • Wiring harness
  • External thermostat/receiver
Full explanation for 83
103

Your boiler is having trouble communicating with an optional safety device, which is causing it to stop operating as a precaution.

High Engineer only £120-250

Parts commonly replaced

  • Safety Assist module
  • Wiring harness
  • PCB
Full explanation for 103
125

Your boiler has detected that the water inside the heating system is not circulating properly and has become dangerously overheated.

High Engineer only £150-300

Parts commonly replaced

  • Circulation pump
  • Flow sensor
  • Diverter valve

Related codes

Full explanation for 125
135

Your boiler has stopped working because it cannot light itself, likely due to a lack of gas or a failed internal sensor.

High Engineer only £120-250

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed Circuit Board (PCB)

Related codes

Full explanation for 135
Blockage

Your boiler has sensed it is getting too hot too quickly and has shut down to prevent damage, usually because water isn't moving through the system properly.

High DIY-safe £120-350

What to check first

  1. Check that all radiator valves are fully open
  2. Ensure the boiler pressure is between 1.0 and 1.5 bar
  3. Press the Reset button for five seconds
  4. Listen for any unusual banging or gurgling noises

Parts commonly replaced

  • Circulating Pump
  • Main Heat Exchanger
  • Flow/Return Temperature Sensors

Related codes

Full explanation for Blockage
Flame Failure

Your boiler has stopped working because it failed to light or lost its flame during operation, so it has turned itself off for safety.

High Engineer only £120-250

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Control PCB

Related codes

Full explanation for Flame Failure
Overheat

Your boiler has detected that it is getting too hot and has automatically shut itself down to prevent damage.

High Engineer only £120-250

Parts commonly replaced

  • Pump
  • NTC Thermistor
  • Diverter Valve
  • Primary Heat Exchanger

Related codes

Full explanation for Overheat
Repair

Your boiler has detected an issue with a temperature sensor and has stopped working to protect itself.

High Engineer only £90-160

Parts commonly replaced

  • NTC Thermistor
  • Flow Sensor
  • Wiring Harness

Related codes

Full explanation for Repair
Status & diagnostic codes

12 operating states & engineer diagnostics

Not faults — these are normal operating states. Tap to expand.

Indicator Neons Status Your boiler is displaying warning lights to signal an internal fault that is preventing it from operating correctly. 2A Diagnostic Your boiler has stopped working because an internal safety fuse has blown, cutting off the power to its main control systems. 3A Diagnostic Your boiler has detected an issue with the power supply fuse, which prevents it from operating safely. 24.0kW Status Your boiler is currently operating at its maximum power output, which is a normal status display rather than a fault or an error code. 100 Status Your boiler is currently running at its absolute maximum power setting, which is usually a temporary state during a manual test or a specific system calibration process rather than a broken part. 100mm Diagnostic Your boiler has detected that the flue pipe is set to the wrong size or is incorrectly configured for your specific model, which prevents it from safely venting exhaust gases. 154 Diagnostic Your boiler has detected that the water is circulating at an unusual speed or temperature difference, often caused by a blockage or a pump issue. 170W Diagnostic Your boiler is experiencing a problem with its internal electrical supply or energy usage, which is preventing it from running safely. C 12 Status Your boiler's electronic control system is failing to recognise its own model configuration, which means it cannot operate correctly. C 32 Status Your boiler's computer has lost track of which specific model it is, preventing it from starting up correctly. G20 Status Your boiler is signalling that it is unable to detect or receive a steady supply of natural gas to ignite the flame. IPX4D Status This code indicates that your boiler has detected a potential issue with its weatherproofing or electrical integrity, meaning it has automatically shut down to prevent a short circuit or damage.