2001 Nissan Sentra transmission code PO730

I started my car this morning and when I put the car in drive it died and check engine light came on. I took it to advanced auto part for a scan and it said knock sensor malfunction and po731 1 gear ratio incorrect. They cleared the codes and I took it to a auto shop. They called me and said that they said no codes besides the knock sensor showed and they told me to put a contioner in for the transmission. I had a trans flush a month ago. Would the codes not come up if agian if they cleared them at the advanced auto parts? I'm know there is something wrong with the car but they cannot find it. They said to come back if the check engine light comes back on but I don't want to wait that long incase it causes more damage. Was there nothing else they could do to to see what is wrong with the car?