I just recently bought a 91 HF with a D16y7 swapped motor. After driving it for a few days i began to think it was running rich and to top that off the check engine light comes on now and then. When i start the car its fine but after driving for some time it will come on. I was clueless so i took it to a shop and he looked at it and basically told me it was probably the o2 sensor. originally i thought this was the problem and it still may be. the thing i'm really confused about is there's supposed to be 2 o2 sensors one on the exhaust manifold and one around the catalytic converter but i can only find one near the cat. any help would be appreciated

 

i think i may have figured it out