External Monitor shuts off when lid closed

I have a Latitude C400 mated to a port replicator with an external display.  When I close the lid of the notebook the external display turns off; using fn + f8 doesn't seem to help.  It didn't  use to work this way: I could close the notebook and the external display continued to work.

Thanks, Mike

RMG,

Thank you for using the Dell Community Forum.
The following may help.
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Question:
How do I keep my external monitor from turning off when I close the LCD lid?

Answer:
Go into Control Panel, Power, look for a setting that states:
"When I close the lid of my portable computer"
Set this setting to "Do Nothing".
Click Apply and Ok.

--or--

Start - Settings - Control Panel - Power Options - Advanced tab - Power button box - When I close the lid of my portable computer - set it to Standby
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Hi Carol,

The power setting your recommend is and has been the one in use.; the setting is set to None for action when the lid is closed.  Note that the computer does not turn off when I close the lid, only the external (and internal) display.

Thanks, Mike

RMG,

You may want to try uninstalling and reloading the latest video drivers.
As well as upgrading to the latest bios revision.