HP EliteBook 840 G3 - Manual (Page 26)

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Component Description
(1) Docking connector Connects 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.
16 Chapter 2 Components