Lady Heather has a command that lets you set the DAC voltage.  It's in the "D" menu.  Depending upon the firmware, you may need to disable disciplining first (also in the "D" menu).  If the DAC  command works to change the OCXO freq,  the osc EFC and DAC are probably OK.  If it does not, I'd suspect the DAC or the  osc EFC input... or perhaps your firmware does no support the DAC setting command.