Trucks been getting like 7-8mpg around town 10 on the hwy, and have noticed a lack of power. Past week or so it's started running rough. I changed plugs wires cap and rotor about a month ago trying to help with the mileage, changed the o2 sensors back in august maybe less than 8,000 miles. Plenum, cat? Any Ideas? Its an 02 2500 with the 5.9, auto, 4x4. Even hooked the superchips back up to it and ran 91 and still wasn't any better. I was getting an avg of 14-16mpg both city and hwy after i canged the o2's then all of a sudden it just dropped.

 

What you already said: I would check the catalytic converter first. If that tests out fine at a shop, I'd check the plenum.