O2 sensor or catalytic converter as last resort. fuel pump and or the pickup in the fuel tank. what color is the tail pipe? darker/sooty leads to an O2 sensor which gives you crappy gas mileage, hard starting, sluggish (to say the least) off the line.
BUT
the least expensive way to trouble shoot any mechanical (retired wrench here) is to pay a professional to run a comp code scan for you. pay the diagonisis fee and go from there.
you'd be surprised how much you save vs. replacing anything you think might be the cause.