the diagnostic code they talk about in that tuning thread causes trouble when it doesn't recieve the "proper" signal from the rear O2 sensor. (a.k.a. removing the sensor or running a sim) the P0420 code is thrown when the CAT is running too far out of spec. A P0420 code will cause the PCM to go into limp mode, unless I'm mistaken, and will cause poor gas mileage. They are actually two different problems that cause the same symptoms...rich running engine and poor MPGs.