Check engine light 2000 GMC

[71 super]

I have 2000 GMC 5.3 and have a couple of questions. The engine light keeps tripping intermitently and codes show 173 174 P. According to manual I have O2 sensors should have fixed issue (down stream) I replaced both sides but the light still trips so what is the deal?? Second I have tick/clatter at first start up if truck has been sitting for a while. I bought truck new has 87,000 miles and has had synthetic oil entire life and delco filters. Has any one had the same issues??? Last question at idle gas needle goes nuts but once rolling no issues. I really like truck but this crap is aggrevating. Anyone have a suggestion(s). Thanks 71 super

[cheyenne402]

I have a 2000 Silverado 2500 4x4 w/ 6.0, it has the same noise at start up. We were told by the dealer that it is common w/ these engines (piston slap). It goes away shortly after start-up. As far as the gas needle, it is probaly the sensor on the fuel pump. Smarter to replace whole pump assembly w/ ac delco unit. Also, probably easier to remove bed to do the job. I am sure someone else might have some suggestions. Good luck.