1996 Mercury Villager code P0135

Computer problem
1996 Mercury Villager 6 cyl Front Wheel Drive Automatic

the check engine light came on went to inspection told me code P0135 came up on their computer before I went to inspection we replaced the oxygen sensor on the exhust manifold all the way in the front of the car the light was off until I turned off the car and once I restared the car the light came bagk on should I replace all the o2 sensors I was told there are 3 more and I also wanted to know if you had a illustration of where they are located