Just picked up a 2003 Impala last night with a 3.4 in it. I was told it gets a P1811 code before I bought it, and since it wasn't registered, I just bought it with no test drive. Stupid me. After towing it home, I went to back it up over the tow trucks wheel lift arms. When I did, it slipped in reverse. It moves in reverse no prob, but under a moderate load, it slips. On a test drive around the block, 1st worked good, second seemed to slip though, and it would shift right into 3rd (which did not slip). Couldn't get it fast enough to check 4th. Anyone have any ideas? I see why the P1811 is set now!