Common problems guide

The most common Alpha CB50 problems

The faults most likely to send a Alpha CB50 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's internal thermometer is failing to send reliable signals, so the system has shut down to prevent it from overheating.

Est. cost
£120-220
Parts
NTC Thermistor, PCB
Full guide for A Flashing
02
B Flashing High Engineer

Your boiler has detected that it is getting too hot and has automatically shut down to prevent any damage or safety hazards.

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

Your boiler has stopped working because it failed to ignite the gas burner, meaning you will currently have no heating or hot water.

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

on the Alpha CB50

NTC ThermistorPCBOverheat thermostatPumpDiverter valveHeat 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 CB50

Related guides

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