Common problems guide

The most common Alpha CB28 problems

The faults most likely to send a Alpha CB28 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.
3 documented codes
3 most common
0 DIY-safe checks
3 engineer needed

The 3 most common faults

01
A Flashing High Engineer

Your boiler is struggling to accurately read its own internal water temperature, which means it has safely shut down to prevent overheating.

Est. cost
£120-180
Parts
NTC Sensor, Wiring harness
Full guide for A Flashing
02
B Flashing High Engineer

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

Est. cost
£120-250
Parts
Overheat thermostat, Central heating pump, Diverter valve
Full guide for B Flashing
03
B Illuminated continuously High Engineer

Your boiler has attempted to start several times but has been unable to successfully ignite its internal flame.

Est. cost
£120-250
Parts
Ignition electrode, Gas valve, Printed Circuit Board (PCB)
Full guide for B Illuminated continuously
Parts most often replaced

on the Alpha CB28

NTC SensorWiring harnessOverheat thermostatCentral heating pumpDiverter valvePrimary heat exchanger

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 Alpha 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 3-code list for the Alpha CB28

Related guides

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