Oxygen sensors
 

My mechanic only has a hand held diagnostic readout to try to fix my car. It has repeatedly indicated that the oxygen sensors were bad (check engine light was on) and I replaced the first one in front, second one in the rear and now am set to replace the second front one as his unit keeps indicating a problem and the light keeps coming back on. Are you supposed to replace these sensors in sets as in doing the front two together and the rear ones together? Could it be that my car's computer is out-of-whack and can this be fixed? Is there 4 sensors total or more?
Any help would be appreciated. Thanks!

RE: Oxygen sensors
 

What year vehicle? How many miles? 1991-1995 used O2's in the exhaust manafold(s) only. 1996-Present uses O2's in the exhaust manafold(s) and after the converter(s). What codes are being activated/pulled?