Dell OptiPlex 755 User Manual page 195

Replacing the Chassis Intrusion Switch
1. Gently insert the switch from underneath the metal bracket into the square hole in the bracket, and then slide the chassis intrusion switch into its slot
until you feel it snap securely into place.
2. Reconnect the cable to the system board.
3. Replace the computer cover (see
Resetting the Chassis Intrusion Detector
1. Turn on (or restart) your computer.
2. When the blue DELL™ logo appears, press <F2> immediately.
If you wait too long and the operating system logo appears, continue to wait until you see the Microsoft® Windows® desktop. Then shut down your
computer and try again.
3. Select the Chassis Intrusion option and then press the left- or right-arrow key to select Reset. Change the setting to On, On-Silent, or Disabled.
NOTE: The default setting is On-Silent.
4. Save your BIOS settings and exit system setup.
System Board Components
Replacing the Computer Cover).