Alpha Cd25 Fault codes & diagnostics

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

Combi Natural Gas 25 kW 2008-2015 Discontinued
GC Numbers 47-532-2447-532-3747-532-39

Replaced by: E-Tec 28

Download the Alpha Cd25 manual The official installation & service manual (PDF) — the exact document these fault codes were verified against. 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 →

Just want the most common Alpha Cd25 faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes

All 5 documented codes

Tap any card for details

LED / flash-pattern codes
A Flashing

Your boiler's internal thermometer is sending incorrect or missing information, which has caused the system to stop working for safety.

High DIY-safe £90-160

What to check first

  1. Turn the boiler off at the main electrical switch.
  2. Wait for 30 seconds.
  3. Turn the power back on and press the 'Reset' button.

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring harness
Full explanation for A Flashing
B Flashing

Your boiler has shut down because it has reached a temperature that is too high, often caused by a blockage or a pump issue.

High DIY-safe £100-£350

What to check first

  1. Check that all radiator valves are fully open
  2. Ensure the system pressure is between 1.0 and 1.5 bar
  3. Turn the selector switch to 'Reset' for a few seconds and then back to 'On'

Parts commonly replaced

  • Circulating Pump
  • Overheat Thermostat
  • Heat Exchanger (de-scale)
Full explanation for B Flashing
Neon A flashing

Your boiler has lost the ability to read the temperature of the water inside it, so it has shut down to prevent overheating.

High Engineer only £120-200

Parts commonly replaced

  • NTC Temperature Sensor

Related codes

Full explanation for Neon A flashing
Neon B flashing

Your boiler has automatically shut itself down because it detected that it is running too hot, possibly due to trapped air or a blockage in the system.

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • Primary flow switch
  • Pump

Related codes

Full explanation for Neon B flashing
Digital codes
B Illuminated continuously

Your boiler has stopped working because it failed to light the internal flame, resulting in no heating or hot water until it is successfully restarted.

High DIY-safe £120-250

What to check first

  1. Turn the selector dial on the front of the unit to the 'Reset' position.
  2. Wait for 30 seconds for the boiler to click and attempt the ignition sequence again.
  3. If the boiler fails to ignite repeatedly, check that your gas supply is turned on and your gas meter has credit.

Parts commonly replaced

  • Ignition electrode
  • Gas valve
  • Printed Circuit Board (PCB)
Full explanation for B Illuminated continuously
Status & diagnostic codes

1 operating states & engineer diagnostics

Not faults — these are normal operating states. Tap to expand.

Neon D Status Your boiler is operating normally and is receiving a steady supply of electricity.