Common problems guide

The most common Main Combi 24 problems

The faults most likely to send a Main Combi 24 into lockout — with plain-English causes, what to check first, estimated repair costs and whether you need a Gas Safe engineer.

This model is discontinued. Parts can be harder to source and expensive. If repair costs are mounting, a new A-rated boiler may be the smarter choice.
13 documented codes
6 most common
2 DIY-safe checks
4 engineer needed

The 6 most common faults

01
15 High DIY-safe

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

What to try first

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

Est. cost
£120-200
Parts
Gas valve wiring harness, Gas valve assembly, PCB
Full guide for 15
02
Blockage High DIY-safe

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.

What to try first

Check that all radiator valves are fully openEnsure the boiler pressure is between 1.0 and 1.5 barPress the Reset button for five secondsListen for any unusual banging or gurgling noises

Est. cost
£120-350
Parts
Circulating Pump, Main Heat Exchanger, Flow/Return Temperature Sensors
Full guide for Blockage
03
20 High Engineer

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

Est. cost
£150-300
Parts
Printed Circuit Board (PCB), Wiring harness, Gas valve
Full guide for 20
04
28 High Engineer

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

Est. cost
£150-250
Parts
Flow NTC sensor, Return NTC sensor, Wiring harness
Full guide for 28
05
40 High Engineer

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

Est. cost
£150-300
Parts
Printed Circuit Board (PCB), Wiring harness
Full guide for 40
06
103 High Engineer

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

Est. cost
£120-250
Parts
Safety Assist module, Wiring harness, PCB
Full guide for 103
Parts most often replaced

on the Main Combi 24

Gas valve wiring harnessGas valve assemblyPCBCirculating PumpMain Heat ExchangerFlow/Return Temperature Sensors

Based on parts cited in our fault code database. Your engineer will confirm what's actually needed after diagnosis.

Call a Gas Safe engineer if…

  • You can smell gas or see signs of a leak
  • The Main shows an Emergency or High severity code
  • The boiler keeps locking out after repeated resets
  • You've tried the DIY checks and the fault hasn't cleared
  • There's visible water leaking from the boiler
  • The flame is yellow or orange instead of blue
See the full 13-code list for the Main Combi 24

Related guides

Boiler losing pressure → No hot water → When to replace → Symptom checker →