Alright guys need some help on this one... Got a 98 f150 4.6 i got the vehicle and added a KN FIPK i then followed that by removing all four catalytic converters and moving to a single in dual out Flowmaster... I got no codes for a year and a half and the truck drove perfectly. 3 months ago one of the welds broke so i was getting alot of sound and carbon and finally after driving around with it like that for a month i got a code I dont remember what number it was but i believe it said bank one sensor to lean. I then did the following mods Gibson headers and a custom built 2 1/4 in exhaust going into a dual in dual out Flowmaster. I also decided to go with two OBD ii O2 bypass on the two downstream sensors. I removed the 0bd 11 bypass and reset the computer and still got these codes.

PO 136 Downstream heated 02 sensor fault bank 1
PO 156 downstream heated 02 sensor fault bank 2

Any ideas? Do i just need to replace the 02 I looked at them with the Scanner and they seem to be operating in the right voltage range. The bank two sensor was really slow though Im assuming thats the pass side. And yes i do realize that they will read slower then the front. Also if i replace the 02 can i go with an aftermarket maybe bosch or borg warner brand or should i stick with motorcraft?

Also just out of curiosity anybody can figure out why i didnt get a catalytic convertor out of spec code?????

 

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Since you're not running any cats then your computer will throw codes.
Those O2 sensors are telling you computer that something isn't right... not sure why they didn't throw codes earlier.