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Originally Posted by jwichman
Had the same symptoms you describe with my 2000 Century at 50K miles. Buick dealership replaced my PCS for $600, lasted about 13 months, just out of warranty, and started the harsh shifting and whine again. Since I drive 90% highway miles, I decided to live with it. Now at 200K miles I still have the intermittant shift problem however tranny still working otherwise.
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That is a common problem to get the harsh shifting and whining back even after a PCS replacement. GM revised the PCS solenoid and portions of the valve body on 2003 and up GM cars with the 4T65E autotransaxle. But the part(s) cannot be retrofitted.
Most of the time all you have to do is turn ignition off for a few minutes then restart and the problem will disappear for awhile. Very intermittent. That is because the DTC that sets P1811 is cleared. Maximum adapt/long shift. When P1811 sets it disables shift adapts and commands high tranny line pressure which causes the shifting problem and whining sound.
Some people claim that either Seafoam or Lubegard tranny additive helps while the problem persists.