Dell Dimension E520 Fan Problem

hi, there... i'm facing fan blow in full speed problem sometime.
whenever i open up casing cover, and put it back.  the fans keep blow in speed without stop (pc can't even start but fan run in speed), i try remove cover... it back to normal

this issues sometime occur sometime not, i try few time and checking cable so on... (got luck back to normal)
my diagnosis tooll light on at 1,2,4  i think

so, could someone help me, what does happen? everytime this case happen, without cover 100% work.