BoilerFaultCodes Neon A (Flashing)
Temperature sensor fault
Alpha CD S Fault codes & diagnostics
6 fault codes with plain-English explanations, severity ratings, DIY guidance, and repair cost estimates.
System Natural Gas 12-28 kW Discontinued
Download the Alpha CD S manual
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. 6 Unrated
Just want the most common Alpha CD S faults?
The top 6 problems in plain English — with causes, DIY checks and repair costs.
Fault codes
All 6 documented codes
Tap any card for details
LED / flash-pattern codes
Neon A (Flashing) and B (Illuminated continuously)
Pump fault or restricted flow
Neon A and B (Flashing alternately)
System pressure is very low and re-pressurisation is required
Neon A and B (Flashing simultaneously)
Blocked flue or fan fault
Neon B (Flashing)
Overheat thermostat has operated
Digital codes
Neon B (Illuminated continuously)
Burner has failed to light