Knowing what fault codes are present will help you diagnose the problem. If you have no way to retreive codes, there are still a few things you can check that might help. When you shift from N to D, does it slam into gear? If so, only the mechanical part of the shifter is working. Does the gearshift display in the instrument cluster indicate the correct gear selected? If not, the electrical part of the shifter is at fault. If the car doesn't slam into gear, and the gearshift display is accurate, check the stop-lamp switch.