P0161 Code on 2000 4.8

bought the code reader tool, www.codereader.com, had that lamp on for few days, bought the tool, cleared it, and everything works fine, no check engine etc, but i still get 600km out of 140liters of city driving with about 500-1000 lbs of load...., normal? Should i get new o2 sensors? btw, i fill up my truck every two-three days, so its a work rig...