Dell A940 All In One Personal Printer - Manual (Page 16)

Auto detect is set but not working properly
Make sure the glass is clean.
Turn off Auto Detect and specify the size of the original.
Fax function does not work
In order for the FAX button to function properly:
Your computer must be connected to a data/fax modem.
The data fax modem must be connected to an active analog phone line.
The printer must be connected to the computer with a USB cable.
The memory may be full. Wait until the faxes in memory have been sent, and then send the
current fax again.
NOTE: You cannot fax with a DSL (digital subscriber line) or cable modem.
See "Faxing" in your Owner's Manual for more information.