Nissan Repair: o2 sensors, nissan altima, oxygen sensors

Question
well, actually autozone and advance only test cars that are 1996 and newer. My check engine light has been on for a while, and i noticed I get horrible gas mileage. and when going up hill, it goes slow. someone suggested it may be my o2 sensor. it made sense to me, just don't know where they are located, isn't the cat converter under the car?
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Followup To

Question -
where is the o2 sensor located on 1995 nissan altima se? also, could a bad sensor cause your check engine light to come on?

Answer -
Kymberly,

There are two.  The first is in front of the converter and the second is behind the converter.  Have Autozone or Checker check the code for free and they will let you know which one it is.  That is, if that is it.  If you get some other kind of code let me know.

Answer
Kymberly,

All automobiles 1993 and newer are OBDII.  So, if they will not check your codes there is a way to perform a self diagnostics using the internal computer on the car.  You will need a Haynes manual to describe the procedure.  If the car is getting bad mileage and the check engine light is on the car is in what is known as limp mode.  That is, the car has a pre-programmed setting to get the car to the service department without damage to the engine.  What this does is retard the timing and increase the amount of fuel to keep the car from pinging.  So, you get bad performance and bad mileage.  I would not suggest replacing parts until I know what the codes are in the computer.  Yes, the oxygen sensors are under the car and are screwed into the exhaust system and there is one in front of the converter and one behind it.  The one in the back only rats you out if the converter is removed.  The forward one is the one that monitors the oxygen that is left in the exhaust gas.  It only provides limited information to the computer so that is why I think you have something else wrong with the car.  I would start with a tuneup that included plugs, wires, cap and rotor.  Also, change the fuel filter and air filter along with the PCV valve.  The car is ten years old and I do not know what has been done to it so that is why I like to baseline the car and then I can look at other issues.  Remember that if the gas cap is left loose it will set a code for the evaporative control system.  You could disconnect the battery for about five minutes and get clear the codes and see if they stay gone.  If it comes back then you will know that the code is hard setting and something really has to be looked at.  But, again, I would check the codes first.