Alpha CB28 Fault codes & diagnostics

8 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.

Combi Natural Gas 28 kW 1997-2005 Discontinued
GC Numbers 47-532-1647-532-1747-532-1447-532-15

Replaced by: CB28 Plus

Download the Alpha CB28 manualThe official installation and service manual (PDF), the document these fault codes were read from. PDF

This boiler uses LED flash patterns

Count the flashes in one repeating group before the pause. That number is your fault code. Do not reset until you have noted it. How to read fault codes →

Severity at a glance

Each cell = one fault code. Hover to identify.
1 Emergency 4 High 3 Low
Just want the most common Alpha CB28 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 8 documented codes

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LED / flash-pattern codes
Neon A and B Flashing on and off alternatively

Your boiler has temporarily stopped working because the water pressure inside the system has dropped too low.

High DIY-safe

What to check first

  1. Locate the filling loop under your boiler or near the hot water cylinder.
  2. Open the valves slowly to let water into the system until the pressure gauge reads between 1.0 and 1.5 bar.
  3. Close the valves firmly and check the boiler display or reset the unit if necessary.

Parts commonly replaced

  • Filling loop valve
  • Pressure relief valve
  • Pressure gauge

Related codes

Full explanation for Neon A and B Flashing on and off alternatively
Neon A Flashing on and off

Your boiler has temporarily shut down because it got too hot inside, but you can usually fix this by simply resetting the system.

High DIY-safe £100-250

What to check first

  1. Check that your home's radiators are turned on and TRVs are not all closed, as this restricts water flow.
  2. Locate the selector switch on the front of your Alpha CD28 boiler.
  3. Rotate the selector switch to the reset position (position 3) and hold it briefly to clear the fault.

Parts commonly replaced

  • Overheat Thermostat
  • Pump
  • PCB

Related codes

Full explanation for Neon A Flashing on and off
Neon B Flashing very slowly (every 5 seconds)

Your boiler is receiving power normally and is currently in standby mode waiting for a heating or hot water demand.

Low DIY-safe

What to check first

  1. No action is required as this is a normal operational status light indicating the boiler is powered on.
Full explanation for Neon B Flashing very slowly (every 5 seconds)
Digital codes
Neon A Illuminated continuously

Your boiler has stopped working because the burner failed to start up safely.

High DIY-safe £80-250

What to check first

  1. Locate the selector switch on your boiler.
  2. Rotate the switch to the reset position and wait for about 30 seconds.
  3. Allow the ignition sequence to restart automatically.

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed Circuit Board (PCB)
Full explanation for Neon A Illuminated continuously