I own a 2003 Audi a4 1.8t. Recently the check engine light turned on. I went to check it out and it tells me "System is too lean" I forgot the number. I did notice it is losing power more specifically not boosting. Some people told me it may be a vacuum leak but not sure where. If someone can help me out just a lil it would be great.

 

ITs likely a vacuum leak. You have thin flexible hoses running through the engine bay, probaly 1/2" outer diameter with a fabric braid on the outside. They dry up and crack with age. Get under the hood when the engine is cold and look around. Follow the hoses with your fingers and look for cracks and worn spots. Whenever I found holse in them, they were usually at or near the ends

There are also plastic check valves located throughout teh engine bay. If you don't know what those are, just look around the bay for plastic pancake-like things as hoses route their way around. Check those for cracks and breakage.

If you have done all of that and still no luck. Look up the tennis ball method for finding vacuum leaks.