The back story is like this. I got a 2004 V6 Mazda 6 almost 2 years ago. The engine has been shaking since I got it. Inspected by a dealer and one other mechanic and they think the catalytic converters went bad. The cats for this car are very expensive and probably cost couple thousands just for the parts. I took it to a backyard mechanics instead and he doesn’t agree with the cat problem. And we also spot couple vacuum leaks. Actually at least three leaks there, including two broken bolts holding the intake manifold. But after fixing the leaks, the car improved a bit but still shaking. It seems to me that this will be a “long-term battle” and so I decide to work on it on my own whenever I find time. And I also try to keep track of what I have done here.
I realize that the car was quite poorly maintained as I inspect it more closely. Besides the three leaks I mentioned above, many hose clamps were missing. I tried to reinstall as many clamps as I can and I decided to replace PCV valve also. In any case, the part is cheap (6 bucks for mine) and the job seems manageable to a beginner like me. I followed this video to locate the PCV valve. But eventually I couldn’t get the old one out like the video said. I had to remove the intake manifold to reach it (suggested by the workshop manual as well).
After putting everything back together, I got code P1410 instead. Car is running open loop and still shaky. It looks like there may be some wiring issues. Probably should replace the connectors to the EGR valve. But I will continue the story next time.