Common problems guide

The most common Alpha CD S problems

The faults most likely to send a Alpha CD S 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
2 DIY-safe checks
4 engineer needed

The 6 most common faults

01
Neon B (Illuminated continuously) High DIY-safe

Your boiler has tried to start up but failed to ignite the gas burner, so it has safely shut down to prevent any issues.

What to try first

Check if your gas supply is working by testing another gas appliance like the hob.Locate the reset button or switch on your boiler and hold it down for 5 to 10 seconds to try restarting the ignition sequence.Check that your condensate pipe outside is not frozen or blocked during cold weather.

Est. cost
£90-250
Parts
Ignition electrode, Gas valve, PCB
Full guide for Neon B (Illuminated continuously)
02
Neon A (Flashing) High Engineer

Your boiler has lost the ability to read its own water temperature correctly, so it has shut down to prevent overheating.

Est. cost
£120-220
Parts
NTC Thermistor, Temperature Sensor
Full guide for Neon A (Flashing)
03
Neon 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, Pump, Diverter valve
Full guide for Neon B (Flashing)
04
Neon A and B (Flashing alternately) High DIY-safe

Your boiler has temporarily stopped working because the internal water pressure has dropped too low and needs to be topped up.

What to try first

Locate the filling loop valves underneath your boiler.Open both valves slowly to let mains water into the system until the pressure gauge on the front reads between 1.0 and 1.5 bar.Firmly close both valves and check the gauge holds steady.

Full guide for Neon A and B (Flashing alternately)
05
Neon A (Flashing) and B (Illuminated continuously) High Engineer

Your boiler has detected an issue with water circulation, meaning it cannot properly move hot water through your system and has safely shut down to prevent overheating.

Est. cost
£150-350
Parts
Central heating pump, Automatic air vent, Flow switch
Full guide for Neon A (Flashing) and B (Illuminated continuously)
06
Neon A and B (Flashing simultaneously) Emergency Engineer

Your boiler has detected a potential blockage in the exhaust flue or a problem with the internal fan that removes dangerous gases safely.

Est. cost
£150-£350
Parts
Flue terminal, Boiler fan, Air pressure switch
Full guide for Neon A and B (Flashing simultaneously)
Parts most often replaced

on the Alpha CD S

Ignition electrodeGas valvePCBNTC ThermistorTemperature SensorOverheat thermostat

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 6-code list for the Alpha CD S

Related guides

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