So I went to change my upstream o2 sensor due to the code saying it was bad. I went to replace it with the denso 234 4124 but when I went to do such, the replacement fit the downstream in the cat and not the upstream. So I think that maybe the previous owner might have put them on backwards. Is there any way to tell? Is the upstream supposed to have the female end or the male end? Thanks guys! It's my first post as I just picked my s2k up a month ago!

 

Front (upstream) o2 sensor has male pins.

Rear has female pins.