Correct, like Pulse_GTO said the PO1 is 512kb and the P59 is 1meg. So you would not be able to to fit it onto a PO1 ECM. PCM Hammer wont allow putting any P59 onto a PO1 or vice versa even though the PO1 is a smaller tune. That is one of the reasons I changed to the P59 from the PO1. The P59 is larger & a faster processer. It did entail rearranging several of the ECM pins from my 2002 vett harness ECM plug to correspond to the correct P59 outputs. But for those who have 2004 / P59 GM ECM'S this will quickly get you going with FLEX fuels or Boosted applications