engine start-up I kept getting the code p0420 ?

i have a 2008 Nissan Altima i kept getting the code p0420 which resulted in me getting another transmission (used) got that put in i can tell the difference got new coils and spark plugs put in around the month of November also got a new catalytic converter put in front and back also. now i keeping getting the code saying it's my downstream o2 sensor i bought 2 new 02 sensors and had them put in also got a code for crankshaft sensor and i bought that new also and had that put in i keeping getting the code for the 02 sensor even after replacing it. every time i start my car up it's extremely loud and you can hear my car running a mile away where do u think that noise is from i thought it was coming from the engine but it's not sound like its coming from the back. every time i start my car up its constantly cranks and cranks and i can hear it try and catch and try and turnover and most of the time it does and sometimes it will sound like a coughing effect and then turns off what do you think the problem is and why I am i still getting the code for m02 sensor after i already replaced both?