one quick simple test to see if the key works is hold the Fn key and press the power button on a clod boot from power off state. If the PSA diagnostics start the Fn button is functional, and we'll need to troubleshoot other issues.
If the PSA diagnostics do not start, then reseat the keyboard (It has a slot connector into the palmrest) and see if that resolves. If still no change, replace 1st the keyboard, and if that doesn't resolve the palmrest.
http://support.dell.com/support/edocs/systems/late6400/en/sm/html/keyboard.htm#wp1179980
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