Car’s Computer Not Recognizing That it’s in Park Not Reverse

Well, this one, while it seems easy, is a bit more complicated than you think. Since your DeVille is in Park but the car’s computer believes it is Reverse, then it is unlikely your vehicle will start, just as you describe. If your car were jammed in Park and your car’s computer believed it, then, at least the neutral-start switch would work when you pressed the brake and your car would turn over.

The problem with your car is two-fold, it seems. The first problem is that the software in your car’s ECU or Electronic Control Unit (shorthand for compute) is really fouled up so your DeVille is having identity problems so that when you put it into Park, it thinks it is in Reverse and so on. The issue here is, more than likely, a sensor, relay or solenoid that is working incorrectly and sending the wrong information to the ECU. For example, if the TPS (throttle position sensor) is incorrectly reporting the positional geography of the throttle then your DeVille just won’t perform correctly. Since it isn’t performing at all and since the computer is reporting the wrong gearing, it looks like the ECU or one of the 15 to 20 sensors that are linked to the ECU may be failing. Look at the error codes that have likely been returned with a diagnostic scanner for better definition of the problem.

The second problem could be in the shift linkage as it seems that your transmission linkage could be putting the vehicle into Reverse instead of into Park. The linkage cables may have stretched or may be binding a bit in their runners so that could be a problem that has to be checked.