97 check engine light problems

All of a sudden my check engine light came on the other day. so i took it to a mechanice and we put it on the diagnostic tester, the only error that came up was a bad 02 sensor. the rear right one was bad, so that was the one that was replaced.. so i leave and 20 mins later it comes on again.. so i took a diagnostic tester from work the othernight and hooked it up and it came up as bad o2 sensor again..so i cleared it, went for about a 25 mile drive before i came home last night and it was good to go.. got up this morning now the damn thing is on again. I have no idea what to do..its annoying the heck out of me. i dont want to pull the bulb out in case something bad were to go wrong, but i want to fix it. any suggestions?

neone?

i've done that about 3 times now..i disconnect it, then hook it back up. the light stays off for about 15 miles then comes on again.. how would the 02 sensor not of been hooked up? the code is p0161..which is the bank 2 sensor..

i dont know about the obd II functions..i'm really lost on what to do..that error is the only one that comes up each time..i've scanned it 2 times after he put the new one in..