BoilerFaultCodes d1 & d2 High
Boiler Fault Code d1 & d2
Also written as D1D2 — same fault.
Reviewed by the BoilerFaultCodes Editorial Team Last reviewed
Fault code d1 & d2 appears across 1 boiler brand and 4 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.
Most common meaning Your boiler has stopped working because the pipe that carries waste water away is likely blocked or frozen, causing water to back up inside the unit. (As reported on Vokera Synergy E)
Pick your boiler brand for the d1 & d2 fix
Source
Information for the d1 & d2 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