Fuel Trim issues

There are a few different reasons on why you could be a getting a check engine light due to fuel trim.  I think I've seen just about every type of issue, and some of the steps I used to resolve/diagnose.

After you've fixed your fuel trim issues, it might be worthy to check and clean your spark plugs.  They could get fouled out if your fuel trim was too rich. 

Last thing, if you want detailed info on fuel trim, and engine diagnostics, I suggest reading through the below document.  You will have a very good understanding of the factory system after reviewing it. 

Advanced Engine Performance Diagnostics

Picture of OEM hoses and parts.