Dell U3821DW 37.5" 21:9 Curved IPS Monitor (Silver) - Manual (Page 73)

Troubleshooting
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3. Move the joystick to highlight the Diagnostic icon and then press the
joystick to confirm. A gray test pattern appears.
4. Carefully inspect the screen for abnormalities.
5. Press the joystick to change the test patterns.
6. Repeat steps 4 and 5 to inspect the display in red, green, blue, black, white,
and text screens.
7. Press the joystick to end the diagnostic program.