Dell OptiPlex 990 (Early 2011) - Manual (Page 44)

7. Lift and remove the system fan out of the computer.
8. Pry up and remove the four grommets from the system fan.
Installing The System Fan
1. Place the system fan in the chassis.
2. Pass the four grommets through the chassis and slide outwards along the grooves
to secure them in place.
3. Thread the system-fan cable into the chassis clips.
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