we have around 100 of this hp laserjet pro 400 in our hospital, and we encounter this error always. This error was due to spool file which was too large for the printer memory to handle. try to clear all the spool file and reset the printer.. try also to install another printer driver, in our case, we tried lower printer driver (HP LaserJet plus).. too many problem with this model... driver installation, corrupted firmware.. and this 79 service error..