Dell OptiPlex 390 (Mid 2011) - Manual (Page 36)

9. Gently pry up the grommets from the system fan and remove it.
Installing The System Fan
1. Insert the four grommets into the system fan.
2. Place the system fan in the chassis.
3. Pass the four grommets through the chassis and slide outward along the
grooves to secure them in place.
4. Connect the fan cable to the system board.
5. Install the fan shelter .
6. Install the drive cage .
7. Install the front bezel .
8. Install the cover .
9. Follow the procedures in After Working Inside Your Computer .
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