Simultaneous 1 Slow / 1 Fast High

Boiler Fault Code Simultaneous 1 Slow / 1 Fast

Also written as SIMULTANEOUS1SLOW1FAST — same fault.

Reviewed by the BoilerFaultCodes Editorial Team Last reviewed

Fault code Simultaneous 1 Slow / 1 Fast appears across 1 boiler brand and 3 models. The exact meaning varies by manufacturer — pick your boiler brand below to see the model-specific fix.

Most common meaning Your boiler is failing to start because its internal safety checks haven't confirmed that the fan is pushing waste gases out of the flue correctly. (As reported on Glow-worm Hideaway 40B)

Pick your boiler brand for the Simultaneous 1 Slow / 1 Fast fix

Glow-worm Simultaneous 1 Slow / 1 Fast 3 models

Source

Information for the Simultaneous 1 Slow / 1 Fast 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