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Buick Park Ave Ultra 1997 GM Code P 1811 Maximum Adapt Long Shift Mode

My 1997 Buick Park Ave Ultra, with super charger, has 115,000 miles. Recently it started shifting poorly when changing gears. Normally the shifts are so smooth you can hardly notice the shifting. The code is P 1811 Maximum Adapt Long Shift Mode. I have searched all over the internet for repair suggestions other than replace the transmission. Can you set the record straight? What can I do to fix this problem? I love the car and with 2 in college and an upcoming daughter’s wedding I do not have the funds to replace the transmission.

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