BoilerFaultCodes A3:-9 to A3:15 Low
Boiler Fault Code A3:-9 to A3:15
Also written as A39TOA315 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code A3:-9 to A3:15 appears across 1 boiler brand and 5 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your boiler is automatically managing its internal pump to protect the system from freezing based on the outdoor temperature. (As reported on Viessmann Vitodens 300-W)
Pick your boiler brand for the A3:-9 to A3:15 fix
Source
Information for the A3:-9 to A3:15 fault code is sourced from the official service / installation manual for each model that displays it. Read more about how we source fault codes →
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026