98 chevy k1500 w/4.3... p0141 code and bad gas mileage

I have a 1998 chevy k1500 with the 4.3L engine. when i brought the code up it said bank 1 sensor 3. Possible bad o2 sensor or fuel pressure. I have not replaced the o2 sensor yet hoping someone has had the same problem and knows exactly what the problem is. thanks!

That code refers to a 02 sensor issue, if it refers to sensor 3 I believe that is behind the cat which has no effect on mileage, unless the cat is blocked of course.

i was incorrect on the p code...its a p0144 which is bank 1 sensor 3. i bought the truck this spring and the previous owner "said" its a new cat but im not sure if it really is or not. i asked around to a few good mechanics in my area and they said to put an exhaust back pressure tester where the 02 sensor goes in the Y pipe and said it should be with in certain spec (which i dont know off the top of my head). Im gonna try the exhaust test and report back. thanks for the reply!