Misfires
 

The entire left bank misfires & the car runs rough. O2 replaced, MAF cleaned, new coils etc, no effects. All fuel injectors seem to be working ok (did a hearing test)

Any ideas? And does anyone know what "Oxygen sensor heater" exactly is?

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Hello Konstantin - If you don't figure it out we do have techs that answer questions on our tech articles. Go here: Porsche Boxster Spark Plugs and Coils Replacement - 986 / 987 (1997-08) - Pelican Parts Technical Article and post your issue with this image in the comment box. One of our specialist will come in with some advice for you. Good luck!

The O2 sensors have a sensor and a heater. There's 4 wires to each. Fuse?

The first thing to check is that you fitted the correct parts. Both the plugs and coils are different in different engine size/year. Some differ by just 1 digit !
Next the connectors on the coils. Unless you lubricate the cover/sheath and push it back to make the connector 'click' into place - that may be an issue.If you don't do this you may have poor connection at the coil and you will swear it is perfect.It isn't.
If you mean Bank 2 (left?? that will be confusing) check the wiring harness. Almost impossible because of the congestion - but use a small ,powerful LED Inspection Lamp. You are looking for rat droppings and maybe loose ground by Cyl #4(brown conductors on a stud).
Next stage is the flailing - swap #4 plug and coil with #1 - does the problem follow? Suggest you find the dwg of the Bank/Cylinder layout to make the diagnostics unambiguous.Using Left/right ,front/back will lead you up the garden path.It really confuses the 996 guys.
Good Luck !

Also make sure the coil packs are seated fully on the plugs, and that the wiring harness connectors are firmly locked on. I had a misfire a few years ago when I did spark plugs, and turns out I didn't seat a wiring connector and thought I had bigger problems. :o

thanks for these suggestions, will check it. Well, I'm not sure what the bank number this is, but what I mean is the bank with cylinders 4-5-6 as on this drawing:

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Bank 1 has cylinders 1,2,3

Bank 2 has cylinders 4,5,6