Did you know the post-cat O2 sensor affects fuel trim? As of lately Mercedes makes it real clear that is does but in fact fuel trim has been affected by the post O2 sensors in Toyota's and Saab's since the late 80's. So as you diagnose-ing vehicles and are having problems with fuel trim just know that a degrading catylitic converter or a incorrectly reading O2 sensor could affect your fuel trim. Manufacturers all have different tunes to change this like GM says 1% correction ford says .5% correction and Toyota is 2% correction +or-. Test have shown that there can be up to 30% fuel trim correction from the post-cat O2 in severe instances. For those who don't know, LTFT=long term fuel trim.

I wouldn't worry about this on your chipped ecu