oxygen sensor elantra

Hi I have a 96 Elantra that is idling very rough and has un-usually high gas consumption. I've brought the car into a dealer and , they tell me that both oxygen
sensors need to be replaced it pointed out P1 and P2 sensors and the MAS ( mass air flow sensor). The MAS sensor was replaced and the car is now driveable I suspect only one O2 is faulty I'd hate to change both at $200 cnd each. On the Hyundai tech site http://www.hmaservice.com/webtech/ running through the trouble shooting steps it says the most likely suspect would be the "down stream" oxygen sensor , now this is where I'm confused , would down stream be after the converter ? Thanks Jose
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yes, down streams after the cat,, id replace both at that cars age

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The downstream oxygen sensor won't affect your fuel mileage. The computer uses it to assess the efficiency of the converter.
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