Can someone please help.

I  have a 06 GMC Envoy Denali XL  that has a check engine light come on and service stability traction control
light on light . I  took it to the dealer I bought it from to get it serviced for a driver side door module recall and
they took my truck to the back for an hour just to be told they didn't have the part. Two days later the check
engine light comes on and I took it back. The service rep tells me that I had a code for  a sensor that keeps the
gas fumes from ejecting out. So I  paid $445.00. 3 days later the check engine comes back on . I take it to
Advance and the same code comes back on now 3 weeks later I have  7 codes on the computer that the dealer
never told me about. Can it be that the computer needs to be replaced. I have a rebulit engine with 45,000 miles
on it but my mechanic gave me list that the codes are  going back to the transmission.  I hate to trade it in
because I just finished paying it off. Please help, I am really confused as to what to do or fix.