I have a 98 Chev K1500 with a 350 vortec. It has 209,000 miles on the same engine. When i first got it, the truck had 190,000. I was getting around 15-17 mpg. It threw a P0420 and P0430 about 3 months ago and now I am getting 9 mpg. I am thinking the cats need to be either replaced or cut off. I have 2 Flowmaster mufflers on there now and would those alone be enough back pressure for that front O2 sensor to get a reading? My friend did it to his vortec but idk if I should. Any Opinions on the idea? I kinda need help.

 

The mufflers are fine I have them on mine with no problems. You probably either have bad 02 sensors or a bad cat.

 

Well I cut the cats off, and holy s*it it's loud. More power but i don't know about mpg or back pressure yet