02 Sensor or Cat??

[Silver_8G]

SES Started as a po150, but before buying 02sensor for that mani. I swapped it with the one in the front. Now after 30-40 miles of driving ses comes back with a po421. Im 99.9% sure its the 02 Sensor from the rear I swapped in. Just wanted a second epinion

[oakrdrs187]

I would replace both, I'm not sure if it's ok to swap them, could be wrong.