What causes Unable to print 51 error?

What causes Unable to print 51 error?

Brother MFC 440CN will not print, copy, scan, fax or turn off by the power button; have to physically unplug. When trying to print it acts as if it is going to before the errir pops up. Checked all possible areas for paper jam. It sounds as if the print head is not engaging not totally sure. Ink volume is fine.