I just put a engine in my car from the junk yard its a 2003 chevy cavalier its runs good and nice and quite but after i drive it for like an hour it starts to act funny like if i sit at a light it feels like it wants to take off by its self and now my engine light came on i checked it with the OBD2 and i got codes PO170 and PO171 what does this mean and what can i do to fix it?