I changed my O2's after about 4 years, the pass. side was decently fouled and the drivers side, almost looked like the new one, just covered in white stuff. The car also feels like it idles and excellerates a little better. Can O2's do that or is it just in my head. Anyway, what caused one side to foul and the other not too?

 

One bank of cylinders running richer than the other.

 

You might find this interesting..

 

Thanks for the link.