Common problems guide

The most common Ariston Microcombi 23 MFFI problems

The faults most likely to send a Ariston Microcombi 23 MFFI 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.
6 documented codes
6 most common
1 DIY-safe checks
5 engineer needed

The 6 most common faults

01
Insufficient water pressure High DIY-safe

Your boiler has stopped because the water pressure inside your heating system has dropped too low to operate safely.

What to try first

Locate the external filling loop, which is usually a small silver braided hose with one or two valves underneath the boiler.Open the valves slowly to allow mains water into the system until the pressure gauge on the front of the boiler reads between 1.0 and 1.5 bar.Close the valves tightly to stop the flow and ensure the loop is disconnected if it is a detachable type.

Est. cost
£80-120
Parts
Filling loop, Pressure relief valve, Pressure sensor
Full guide for Insufficient water pressure
02
A09 High Engineer

Your boiler has detected that water is not circulating correctly through the system, so it has safely shut down to prevent internal damage.

Est. cost
£150-250
Parts
Circulation Pump, Pressure Switch, System Filter
Full guide for A09
03
A10 High Engineer

Your boiler is having trouble controlling the size of its internal flame, which means it cannot safely heat your water or home.

Est. cost
£150-300
Parts
Gas valve, PCB
Full guide for A10
04
A11 High Engineer

Your boiler is struggling to clear exhaust gases out through the flue, so it has safely shut itself down to prevent fumes from building up.

Est. cost
£120-250
Parts
Air pressure switch, Flue fan assembly, Venture pipe
Full guide for A11
05
A12 High Engineer

Your boiler has detected that the water temperature has risen too high and has safely shut itself down to prevent overheating.

Est. cost
£120-250
Parts
Safety thermostat, NTC thermistor sensor, Circulation pump
Full guide for A12
06
Boiler Shutdown High Engineer

Your boiler has detected an internal safety issue and has shut itself down to prevent any damage or danger, meaning you currently have no heating or hot water.

Est. cost
£150-300
Parts
Printed Circuit Board (PCB), Air Pressure Switch, Flame Sensing Electrode
Full guide for Boiler Shutdown
Parts most often replaced

on the Ariston Microcombi 23 MFFI

Filling loopPressure relief valvePressure sensorCirculation PumpPressure SwitchSystem Filter

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 Ariston 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 6-code list for the Ariston Microcombi 23 MFFI

Related guides

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