Tranny problems

[Vukemedo]

Hi I am new on this forum, and have a problem with my car and hope somebody can help me figure out what's wrong with my car. I have a 2004 Porsche Cayenne V6 50k.. I was driving on the highway in Drive.. And when I hit a light I went to accelerate after it turned green and my car was in 6th Gear in Drive.. So I slapped it over to tiptronic and manually shifted down to 1st and the car went into 1st gear so I gave gas and put it back in Drive. Everything was fine until I parked my car, and I shut it off.. I later returned and started my car to make my way home and noticed I had a light come-up on my dashboard, it was yellow and I believe it was check engine or transmission. Anyways so I put the car in drive gave gas and didn't see a difference till I got to 40 mph.. I saw that my car wasn't shifting after 2nd gear and my car was at about 5 1/2 RPM.. So I slapped it in tiptronic thinking I can manually shift it up and the car didn't recognize that I was in tiptronic it just said "D" on the dash. So I called my mechanic and he told me to keep the car under 3 RPM and drive to the shop.. I drove at 3RPM for about 43 miles at speeds ranging from 29-34mph I couldn't go faster or my engine rev'd too high. So my "mechanic" if you want to call him that told me my transmission is fine and it went on "safe mode" he said it was a circuit.. Now 4 days later he is telling me he doesn't know what the problem is.. But I have never had problems with my transmission I've never felt clunks or any loud sounds when changing gears from park to drive or to reverse. And I barely drive in tiptronic and when I did I never felt any gears slip or make sounds either.. I'm very puzzled to what could be the problem with my car.. And does anyone know what I should do about my "mechanic" ?? I shouldn't have to pay for anything if he didn't fix it right?? Thanks.

[RFM]

Let the car check on fault codes with a PIWIS diagnostic tool, this codes are essential for a diagnosis.