Just picked up a 2003 Impala last night with a 3.4 in it. The trans is a 4T65E just like the Bonneville has. 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!

Do a search on P1811. You'll find that some others have had success with a shift kit and some Lucas no-slip fluid added when a service is done to add the shift kit.

P1811 is the max adapt long shift. Meaning the pcm is seeing a very long shift time. This code would only relate to 1-2 shift.

Update. The car seems to work in reverse fine now. I drove it around and got a P0742 code and P1811. Talk about hard shifts! Here'* the problem though. 2nd gear seems like it'* missing. I temporarily disconnected the lockup wire at the harness plug so the Converter clutch stays off. Drives ok, but 2nd is a no go. It shifts from 1st to 3rd. I believe something may have broke during the hard shifts with the converter locked up. Any input?

i got the same ones in my 2000 bonneville p0742 is your TCC switch and P1811 is for max adapt it constantly thinks you are one the highway and boosts the line pressure to max and that is where the hards shifts come from more than likely a bad TCC cheap part about $40 but getting to it is a pain in the rear

Sorry to be the bearer of bad news, but your trans needs an overhaul.

There'* always the much lower cost replacment. I used to be a new only guy..then I had too many cars wtih too many issues and the JY'* are treating me well.

I found a yard semi local that has a 6 month unlimited mileage warranty on parts. They pull for you...and the prices are usually pretty darn good.

That is a good option, but if the replacement unit fails, it is still a PITA to swap it. If the replacement trans has not been recently rebuilt, you may have the same issues as your unit in short order.