Was it a universal sensor you installed or one of the ones designed for our trucks?

Not sure of that makes a difference, but there is a chance. It may be that, and/or the computer/scanner may be, for some reason, finding the sensor is working but not up to emissions standards (not as efficient as it should be).

Maybe try a different brand of sensor. I have had zero issues with NTK sensors (made by NGK).

 

It's from the Fastman; I've heard good things about them. From my scanner, it says the rear sensor and cat is fine; that means it's nothing to do with my sim or cat, right? I've had troubles with my front sensor since before my new cat. I've tried to check my wiring, but it all seems ok. Any other ideas?

 

Maybe see if you can't go to Autozone or borrow a friend's scanner. Maybe your scanner isn't reading it correctly for some reason, and its your scanner or a problem with your scanner. Just a guess, but its free, might as well try it.