A computer system's diagnostic mode was used to ascertain whether or not there were problems or inconsistencies in a starship's main systems, including the transporters. When Miles O'Brien, Data and Deanna Troi, taken over by Ux-Mal criminals, wanted to make sure that nobody could use the transporter to beam them back to Mab-Bu VI, O'Brien took the entire transporter array off line and placed the transporter in a diagnostic mode which would take hours to complete its cycle. (TNG: "Power Play" )