Why is my 2003 saab 9-3 stuck in limp mode?

My car has always had issues. It has about 80k miles on it and all of its issues go back to the transmission. I blew my old transmission over the summer so we got it replaced and its been fine. The transmissions not the problem its the sensors. Even my old transmission has had problems with sensors. Ive been thru 3 speed sensors with the new transmission. Everytime it gets replaced it works for a month sometimes 2 then I have to bring it back to the transmission shop and get it replaced again. They went thru all the wiring and everything and everything is fine and theyre not ripping me off because the owner is my dads absolute best friend. But I just got the sensor replaced again yesterday and it just doesnt do anything they said they test drove it and the car works fine for about 15 minutes when it hasnt been used for a day but it just gets stuck in limp mode and everytime we scan the check engine light it goes right back to the sensor and says the same thing and we just replaced it and have replaced it multiple times. I know saab doesnt have the best reputation and I shouldve sold this thing a while ago but I made it this far I just dont understand why we keep replacing sensors and it keeps acting up and the scanner says its the sensor or wiring and they went thru everything