The idea behind the O2 eliminator is to keep the stock O2 sensor from sending data to the ECU that may butt heads with what your power commander is doing with your fuel mixture, i.e your power commander is making the A/F richer and your O2 sensor sends a message to your ECU to lean the mixture because it "thinks" the mixture is too rich or, vice sersa. The stock ECU has it's own data for controlling the mixture via the time the injectors are open, and the power commander can modify that data by either lengthening or shortening the duration. If the power commander's program wants to lengthen the duration (to add more fuel and therefore make the mixture richer) but the O2 sensor sees the richer mixture and tells the ECU it's too rich, it may try to lean the mixture to get it back to where it thinks it should be. If you were to pull the O2 sensor without an eliminator, it would flag an error code. If left intact, you'd probably never see a problem but you'd also never know if the ECU was tryig to overrun what the Power Comander is doing. I hope this sheds a bit of light on the subject.