A computer system's diagnostic mode is used to ascertain whether or not there are problems or inconsistencies in a starship's main systems, including the transporters. Locking a starship's transporters diagnostic mode, effectively putting it into a diagnostic cycle, will keep any inert matter in the pattern buffer. Ordinarily, inert matter can only remain in the pattern buffer for seven minutes before becoming irretrievably lost, but Montgomery Scott was able to reconfigure a transporter, locking the pattern into a diagnostic cycle to keep his pattern in the pattern buffer for 75 years. His pattern was retrieved with minimal degradation. (TNG: "Relics")
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 offline and placed the transporter in a disgnostic mode which would take hours to complete its cycle. (TNG: "Power Play")