BoilerFaultCodesGlow-worm F16 Fault Code
The Glow-worm F16 fault code means: your boiler thinks it sees a flame before it has even started the ignition process, which is preventing it from running for safety reasons.. It appears on 3 Glow-worm models in our database.
Technical description: Flame detection fault (Flame detected before ignition).
What causes the Glow-worm F16 fault?
This fault occurs when the boiler's sensors detect a flame signal at a time when there shouldn't be one, such as during the initial startup checks. The most common reason is a faulty flame sensing electrode that is sending a false signal, or a carbon buildup that is allowing electricity to leak across the sensor. In some cases, the main control board may be malfunctioning and incorrectly interpreting signals from the burner.
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 F16 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm F16 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