Unable to put arduino mega in DFU mode.!!!

Please someone help i have arduino mega may b its a clone but it have a 16u2 chip its a squre type and 16u2 is written on that small chip but i am still not able to put it into DFU mode. when i short frist two pins of ICSP1 it just disconnects and don't reconnect it vanishes from device manager untill i dont leave the pins unshorted. Is colne having 16u2 chip dont go in DFU mode??