Got a check engine light while driving yesterday. Today I took it to mechanic and he got two errors. 1. O2 sensor faulty (the less important one?) and 2. Cylinder 2 is misfiring. Two questions: How much is O2 sensor and where is it? And second, what is the most common and some less common problems for a cylinder to misfire? Is it a spark plug problem usually? Thanks for the info.

PS: I don't think I can attribute these two problems to DRIVING HARD, necessarily. >

__________________
2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)