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

 

yes, down streams after the cat,, id replace both at that cars age

 

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