What is the balance shaft and is it expensive to replace?

What is your question?

My check engine light came on and I failed my emission test. My CLK350 is a 2008 and I have 124,000 miles, but expected many more based on what it cost. I've taken it to several auto repair shops that work on the vehicle, but no one wants to tackle the engine. The car drives fine - no noises or leaks. I love the car, had planned to drive it longer, but don't want to put the price of a less expensive car into it.

How long has your 2008 Mercedes-Benz CLK350 had this problem?

The check engine light was sporadic initially, but now it's constant.