Alpha CD25C Fault codes & diagnostics

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

Combi Natural Gas
GC Number 47-532-31
Download the Alpha CD25C manualThe official installation and service manual (PDF). 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 →

Fault codes

All 2 documented codes

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Neon A Flashing

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

High Engineer only £120-220

Parts commonly replaced

  • NTC Thermistor
  • Temperature Sensor
Full explanation for Neon A Flashing
Neon B Flashing

Your boiler has temporarily shut down because the internal temperature got too high, but you can usually clear this by simply resetting it.

High DIY-safe

What to check first

  1. Check that your radiators are turned on and TRVs (thermostatic radiator valves) are not all closed, as this can block water flow.
  2. Ensure your boiler water pressure is at the correct level (usually around 1 to 1.5 bar).
  3. Rotate the boiler selector switch to the reset position and hold it briefly to clear the lockout.

Parts commonly replaced

  • Overheat thermostat
  • Pump
  • PCB

Related codes

Full explanation for Neon B Flashing
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