To help you answer your question, I'm going to suggest you look up the name of that code.
Once you do that, you won't be looking for a header or vacuum leak.
It's not an O2 sensor, The O2 sensor is the sensor that is reporting the issue to the pcm, the pcm is then alerting you of it.
It bothers me when guys will say "had this issue for a long time" and "I'm replacing sensors etc" Reason being is..you didn't look up the code, you didn't research it and you don't seem to really care, since it's been happening for a while. Sorry if I'm a bit harsh, but I didn't get laid last night and you are being mean to your car by not researching the issue.
My last piece of advice is to search on your code at Gearchatter.com. Then read the diagnostic aids section for your code. Usually the problem is fixed by checking those aids.
