I bought my Z last may and have been throwing SES 1283 and 1284 both bank 1 and 2 since I had the car.
Car came with megan racing resonated test pipes with nonfoulers, JWT intake, and 5/16 motordyne plenum spacer. and 80k miles.
Everyone told me they were sure its just bad o2 sensors but that sounded like bullcrap because all 4 cant get shot.
I put my Cats back in last night and finally came to realize that the car was simply running so lean it was off the stock map.
A solution some people might want to try if they are getting the same codes as me with breather mods is to get the car tuned. (I wish I had realized that before, it would have saved me alot of money)

-eddie