Alpha Cd25 Fault codes & diagnostics

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

Combi Natural Gas 25 kW 2008-2015 Discontinued

Replaced by: E-Tec 28

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.
16 High 2 Low
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 18 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
E01

Your boiler has tried to light several times but has been unable to successfully create a flame to start heating your water.

High DIY-safe £100-280

What to check first

  1. Check that your gas credit hasn't run out or that the emergency control valve is open
  2. Press and hold the Reset button for 2-3 seconds until the error clears
  3. Check if other gas appliances in your home, like a hob, are working correctly

Parts commonly replaced

  • Ignition Electrode
  • Gas Valve
  • Flame Sensing Lead

Related codes

Full explanation for E01
E02

Your boiler has switched itself off because moving water inside the system has become too hot.

High DIY-safe £90-280

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. Press and hold the reset button for several seconds

Parts commonly replaced

  • Overheat thermostat
  • Circulation pump
  • Temperature sensors (NTC)

Related codes

Full explanation for E02
E03

Your boiler has detected a problem with how it disposes of waste gases, causing it to shut down for safety.

High Engineer only £120-280

Parts commonly replaced

  • Air pressure switch
  • Flue fan
  • Venturi tube

Related codes

Full explanation for E03
E05

Your boiler has stopped working because it cannot correctly measure the temperature of the water in your heating system.

High Engineer only £90-160

Parts commonly replaced

  • Heating Flow NTC Sensor
  • Thermistor
  • Wiring Harness

Related codes

Full explanation for E05
E06

Your boiler has detected a problem with the sensor that monitors hot water temperature, which usually means you will have little or no hot water.

High Engineer only £90-160

Parts commonly replaced

  • Domestic Hot Water (DHW) NTC Sensor
  • Sensor Wiring Harness

Related codes

Full explanation for E06
E10

Your boiler has detected that the water pressure in the system is too low to operate safely, which has caused it to shut down temporarily.

High DIY-safe £0-120

What to check first

  1. Locate the filling loop (usually a silver flexible hose) underneath the boiler.
  2. Open the one or two small taps on the loop slowly until you hear water flowing.
  3. Watch the pressure gauge and close the taps firmly once it reaches 1.5 bar.

Parts commonly replaced

  • Filling loop
  • Expansion vessel
  • Pressure sensor

Related codes

Full explanation for E10
E16

Your boiler has detected an issue with the fan that safely clears away used gas, which has caused the system to stop working for your safety.

High Engineer only £220-£380

Parts commonly replaced

  • Fan motor
  • Main circuit board (PCB)
  • Air pressure switch
Full explanation for E16
E24

Your boiler has detected that the water temperature entering and leaving the unit is too far apart, which usually indicates the water is not circulating correctly through the system.

High Engineer only £120-£350

Parts commonly replaced

  • Circulation pump
  • Flow/Return thermistors
  • Main heat exchanger (cleaning/flush)
Full explanation for E24
E31

Your boiler has lost its connection with your external thermostat or remote control unit, preventing it from receiving heating instructions.

High DIY-safe £120-280

What to check first

  1. Check the batteries in your room thermostat and replace them if necessary
  2. Ensure the wireless receiver is powered on and within range of the thermostat
  3. Restart the boiler using the reset button to see if the connection re-establishes

Parts commonly replaced

  • Remote Thermostat batteries
  • Wireless Receiver unit
  • Room Thermostat
  • PCB (Printed Circuit Board)
Full explanation for E31
Neon A

Your boiler has stopped working because it is unable to accurately measure the temperature of the water inside it.

High Engineer only £120-220

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring harness

Related codes

Full explanation for Neon A
Neon B

Your boiler has automatically shut down because it detected that it was getting too hot, likely to prevent internal damage.

High Engineer only £120-250

Parts commonly replaced

  • Overheat thermostat
  • Pump
  • Diverter valve
  • PCB

Related codes

Full explanation for Neon B
Neon D

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

Low DIY-safe

What to check first

  1. Check if your thermostat or programmer is set to call for heat
  2. Ensure the central heating selector switch is in the 'on' position
  3. Verify that your hot water tap is open if you are testing for immediate output
Full explanation for Neon D