2001 Honda Civic • 100,000 miles

Hi I am not too sure about what is wrong with my car.
First my car was misfiring, so I have already changed the fuel pump, cleaned the fuel system out, changed spark plugs and replaced the upstream o2 sensor. It has stopped misfiring, but now my check engine light comes on after traveling at higher speeds or uphill. When I'm driving in town the CEL goes off. Also, there is a cracking, or popping noise coming from the front driver side of my car, I travel about 40 miles a day so I really depend on my car. If you could help me diagnose this problem that would be great! Thanks.
December 9, 2012.

Have the computer scanned for code/s and also check the CV Joint/boot and wheel bearing on the driver side-

Dec 9, 2012.
I got the codes read, and the only thing that comes up is the oxygen sensor but I already changed the upstream, do you think I would have to change the downstream as well?

What is the exact code it gave you for the O2 sensor

Dec 10, 2012.