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.
4 documented codes
4 most common
1 DIY-safe checks
3 engineer needed

The 4 most common faults

01
E10 High DIY-safe

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

What to try first

Locate the external filling loop, which is usually a flexible silver hose connected between two pipes under the boiler.Open the small valve(s) on the loop to allow water into the system while watching the pressure gauge on the front of the boiler.Close the valve(s) once the needle reaches the green zone, typically between 1.0 and 1.5 bar.

Est. cost
£80-120
Parts
Filling loop, Pressure relief valve, Pressure sensor
Full guide for E10
02
E01 High Engineer

Your boiler is failing to light because it cannot detect a flame, meaning you currently have no heating or hot water.

Est. cost
£150-300
Parts
Ignition electrode, Gas valve, Printed Circuit Board (PCB)
Full guide for E01
03
E02 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
Pump, Diverter valve, Primary heat exchanger
Full guide for E02
04
E03 Emergency Engineer

Your boiler has detected an issue with the ventilation system that clears away exhaust gases, so it has shut itself down to keep your home safe.

Est. cost
£120-250
Parts
Flue pressure switch, Air pressure switch tubing, Fan assembly
Full guide for E03
Parts most often replaced

on the Alpha CB28

Filling loopPressure relief valvePressure sensorIgnition electrodeGas valvePrinted Circuit Board (PCB)

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 4-code list for the Alpha CB28

Related guides

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