Persistent paper jam error 077-900. There is no paper in the printer.

I am getting a persistent paper jam error 077-900 even though there is no paper jammed in the printer.  I have checked the registration sensor and the feed solenoid.  They are both working.  The error code refers to an exit sensor.  Where is this 'exit sensor'? I have never used the fax option on the printer, so it cannot be referring to the sensor there.