Unless you want to pay a whole lot for a transmission scanner, just bring it to a reputable transmission shop or dealer.
If the torque converter is slipping your engine will start to stall and cut out eventually you will be driving and the car will rev in the drive position as if it is in neutral go to a transmission shop and get the tech to test drive your car.
It has low oil or it has serious internal failure. With out a test drive and at leasta 1 hour diagnostic, no further can be determined.
Do a compression test.
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You fill the primary/transmission oil supply with one quart of HD Sport/Trans through either the derby cover or the chain tension test cover.
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My best guess would be for the parking break. Easy way to test, when the car is running just pull the lever or step on the lever and see what light turns on