I'm replacing the catalytic converters on my Express 3500 cutaway box truck, and I'm unsure which downstream O2 sensors connect to which exhaust. Why is this an issue when the O2 sensors are clearly labeled left and right? Well, the wiring harnesses in the rear are placed in a way where the downstream right O2 sensor connects with the driver's side (left) cat, and the downstream left O2 sensor connects with the passenger's side cat. I know this because the wiring harnesses (along with the O2 sensors) are color-coded (white = driver's side, black = passenger's side). At the moment, I have 1 white and 1 black O2 sensor connected to each respective pipe, when in theory each pipe should have its color O2 sensors. To add to the confusion, the cats cross over each other, meaning that the diver's side cat is closer to the passenger side and vice versa with the other cat. Keep in mind that I bought this box truck with the cats already stolen off of it, so I have no frame of reference for how the O2 sensors were set up in the first place. I am also waiting for some exhaust sealant to dry, so I haven't had the chance to run the truck in its current configuration. If someone has any insight into the proper O2 sensor placement, please let me know. Even a reference picture of some sort would be a great help. I've attached a diagram of my setup. Thank you!

 

Unplug one and see what OBDII code you get?