I'm not familiar with the Zenith in particular, but are there other buttons? Such as one for VCR, Aux, etc.?
If not that's fine, but if so there are much more buttons and therefore codes.
Basically, you have to keep trying to find the IR signals than will keep showing up the same to your IR reader. So take the list of 3 digit codes and try them till you find one that works with all the buttons.
On my universal remote, I found that the IR codes for the VCRs worked the best. Also my controler has TV, VCR, Cable, and Aux modes. I programed all these buttons for the same listing of codes (VCR) and then found unique codes for each button.
Hope this helps.