I have a 1990 Deville 4.5L. When I bought this car I was told by the dealer that I had to fix the PRNDL (I trust the dealer, he's family). How do you test for the PRNDL? And how do you fix it? I know the code is E91 but how do you run the diagnostics to find the code? It affects the automatic door locks(they don't open or lock when shifting to park or drive) and the clock on the radio stays on when the car has been turned on then shut off. I have to shut off the clock on the radio by pressing the door button on the outside of the car. It also bangs on the gear when i shift into reverse. Any help?