Common problems guide

The most common Alpha CB24 problems

The faults most likely to send a Alpha CB24 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.

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 Low Engineer

Your boiler is operating exactly as it should be, and the 'B' light simply confirms that the burner is currently lit and heating your water.

Full guide for B Illuminated continuously

Parts most often replaced

on the Alpha CB24

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 CB24