Alpha CD Systematic Fault codes & diagnostics

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

System Natural Gas 12-28 kW Discontinued
Choose your boiler type

The CD Systematic range comes in 3 types

Pick the one you have. The fault codes, GC numbers and manual differ by type.

CD12S 0 codes · 1 GC number CD20S 0 codes · 1 GC number CD28S 0 codes · 1 GC number
Each model has its own manual A family spans several manuals, one per model or generation. Pick your model above and its page has the manual its fault codes were read from.

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

Tap any card for details

Neon A flashing

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

High Engineer only £120-220 Shared across the CD Systematic range

Parts commonly replaced

  • NTC Temperature Sensor
  • Wiring harness

Related codes

Full explanation for Neon A flashing
Neon B flashing

Your boiler has detected that it is running too hot and has automatically shut itself down to prevent damage.

High Engineer only £120-250 Shared across the CD Systematic range

Parts commonly replaced

  • Overheat thermostat
  • Pump
  • Diverter valve
  • PCB

Related codes

Full explanation for Neon B flashing