Your precat 02 reads AFR while your post cat checks cat efficiency... at least on most newer cars. You can go on eBay... heres what you need to install the secondary 02 it in a sense tricks the ecu. You can turn the secondary off with a tune, but that's more than $20.
How's this for less than $20 A few links...
http://www.ebay.com/itm/Oxygen-senso...72e7b2&vxp=mtr
http://www.ebay.com/itm/02-OXYGEN-SE...cb06a3&vxp=mtr