BoilerFaultCodesGlow-worm Simultaneous 1 Slow / 1 Fast Fault Code
Also written as SIMULTANEOUS1SLOW1FAST — same fault.
The Glow-worm Simultaneous 1 Slow / 1 Fast fault code means: 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.. It appears on 3 Glow-worm models in our database.
Technical description: Lighting Sequence-Fan/APS / Check connection & operation
What causes the Glow-worm Simultaneous 1 Slow / 1 Fast fault?
This fault occurs when the boiler's safety system detects that the fan isn't moving enough air to safely expel exhaust gases. The most common reasons are a seized fan motor, a faulty air pressure switch that fails to signal the control board, or small tubes becoming blocked with debris or moisture. Without confirming a clear airflow, the boiler will refuse to ignite to prevent a buildup of dangerous fumes.
This issue is more frequent during the first cold spell of autumn, as fan bearings can seize up after sitting idle throughout the summer months.
Engineer required
This fault requires a Gas Safe registered engineer. Do not attempt gas-related repairs yourself.
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Gas Safe Register →Glow-worm models that show the Simultaneous 1 Slow / 1 Fast fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm Simultaneous 1 Slow / 1 Fast fault code information on this page is sourced and cross-checked against the official Glow-worm service / installation manuals for each affected model.
Reviewed by the BoilerFaultCodes Editorial Team · Last reviewed 27 April 2026