BoilerFaultCodesGlow-worm F.84 Fault Code
Also written as F84 — same fault.
The Glow-worm F.84 fault code means: your boiler is confused because its temperature sensors are giving conflicting readings or are installed incorrectly, causing the system to shut down for safety.. It appears on 6 Glow-worm models in our database.
Technical description: Flow/return temperature sensor fault
What causes the Glow-worm F.84 fault?
This fault occurs when the boiler's internal computer receives conflicting information from the two sensors that measure water temperature entering and leaving the unit. It is usually caused by a sensor that has developed a chemical fault, or because the sensors have been physically swapped or installed on the wrong pipes. Because the boiler cannot accurately track how heat is moving through the system, it shuts down to prevent the primary heat exchanger from overheating.
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 F.84 fault code
Click a model for the full fault code list and model-specific repair notes.
Source
Glow-worm F.84 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