I have been researching like crazy to find a solution, I stumbled upon this and I hope someone can help. I have a 2005 chevy impala with the p1811 code. Driving uphill is a pain I will be cruising at 60 mph and once I start going uphill the mph goes down to about 40 and I have to really give it gas to make it go. I have changed the catalytic converter, new plugs, wires, fuel filter. All made a difference from when I first bought it. I have a shift kit in the garage but I'm not sure if installing it will help. Can someone please for the love of baby Jesus give your thoughts!!!