B7S4CO

Hi everybody, this is my first post on the forum, so forgive me if i am posting in the wrong section. Well last week i bought a 2005.5 audi s4. I had my independent mechanic do a PPI on the vehicle before i purchased it, the mechanic said the car was in great shape, and he saw no reason not to purchase it. So fast foward to this week, and the check engine light came on today while I was driving home. Once i got home i was sitting in my parking spot, and the car was turned on and idleing, then white smoke started coming out of the drivers side of the engine. However all the temperature gages were acting normal, and the smoke did not continue for long after the engine was turned off. My first guess is O2 sensor, but i wanted to see what you guys thought. Thanks