HP EliteBook 840 G3 - Manual (Page 24)

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Component Description
(1) Docking connectors (2) Connect an optional docking device.
(2) Vents (2) Enable airflow to cool internal components.
NOTE: The computer fan starts up automatically to cool
internal components and prevent overheating. It is normal
for the internal fan to cycle on and off during routine
operation.
14 Chapter 2 Getting to know your computer