My 3100cn started showing a 004-321 error code.  I tried the fix of cleaning the silver square on the belt.  See the post by Dell-Bill B 07-04-2007 11:49 AM.     The error code continued so I cleaned the face of the sensor that reads the silver square.  Still getting the 004-321 at start-up I looked at the sensor connector and noticed it was not fully seated.  With a small screw driver I pushed the connector back in place.  The error code went away.

 

To check the sensor, first remove the drum unit.  The sensor is located above the developer units on the left.  The connector is on the left side of the sensor.