I had to install a new set of spark plugs in my Z4 35is last night. I just turned 39,000 miles and on my way home from work on Tuesday my car developed a misfire. The service engine light came on also. I got home and used my hand held scanner to pull the codes . The scanner told me I had a misfire on cylinder 1. I knew then it was a bad plug. I went to my BMW Dealership parts dept. and bought a new set of plugs for $96.00 buck's ! They wanted a $120.00 but, I told them that was to much and was going to look on ebay for a better price. That is when the lower price was offered. When I pulled the plugs they looked bad. They were all black and looked dirty. You could tell it needed new plugs. Got in my Z this morning and she is running like new again ! Thank goodness ! I checked my owners manual and it says plugs will need to be replaced at 45,000 miles. Sooner if you do a lot of city driving. I do a lot of city driving. Anyway just wanted to share this with Y'all . If you notice your Z is running a little sluggish it may need a fresh set of spark plugs. Merry Christmas everybody !