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I bought a 2002 BMW 330Ci from a guy that seems to have overestimated his mechanical prowess. He installed headers without cats, so I expect some codes to be thrown. He told me that he had also replaced all the O2 sensors and spark plugs. Which leads me to my current dilemma. These are all of the codes I'm currently getting: P0057, P0037, P0051, P0031, P0304, P1347, P1343, P1345, P1353, P1351. As far as I understand, P0057-P0031 are telling me that none of the O2 sensor heaters are working, (funny coincidence after he - a DIY mechanic - replaced the sensors. The other codes tell me that all cylinders are experiencing misfires.
Sometimes, after warming up, the car will start running very rough, so I'll plug in my OBD scanner, which is throwing misfire codes at me, and clear the codes. Then it runs fine again.
After researching around, the best I can gather is that I either have an intake vacuum leak, or something is wrong the ignition system itself. In my mind, if clearing the misfire code fixes the issue, it's probably not faulty wiring, otherwise the issue would persist, wouldn't it? I am also led to believe that one or two faulty coils or plugs would not cause all 6 cylinder to misfire. This logic (I have no idea how sound it is) tells me that it is likely a vacuum leak.

In short, I have two questions:
1. Given the information, Do you think that my misfire issue is related to a vacuum leak or The ignition system?
2. What do I do about the O2 sensor issue? Do I need to replace all the sensors? Could it just be a blown fuse somewhere? I have no clue what is happening with them or what to do to fix it.

Any help would be much appreciated!
 

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Has the CCV system been done on this car? I had a litany of codes, including misfires and O2 sensor circuits, and all went away after CCV + replacing failed vacuum hoses leading to the intake manifold.
 
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