Why a car won't start in Park but starts in Neutral?

Updated: July 07, 2021
For safety reasons, the engine in your car can only be started when the automatic transmission is in Park or Neutral position. How does your vehicle know if the transmission is in Park or Neutral? There is a transmission range (position) switch or sensor that in most cars, located on the transmission. It's also known as a neutral safety switch.
Automatic transmission range switchTransmission Range Switch (Sensor)
It detects which gear the transmission is in and sends this information to the vehicle computer, called PCM.

In most cars, the transmission shifter inside the vehicle is connected to the vehicle transmission through a mechanical link, such as a cable or linkage. When you move the shifter inside the car, the cable/linkage moves a little lever connected to the selector shaft of the transmission.

Possible causes

The most common problem with this setup is when the cable or some other part of the mechanism rusts, wears out or becomes loose. For example, see this photo of a rusted shifter cable connection at the transmission side. Sometimes, a transmission shifter inside the car becomes sticky and hard to move too. This restricts the movement of the cable and causes the problem where the shifter inside the car is in Park, but the lever at the transmission range switch (in the photo) does not move far enough. As a result, the transmission range switch cannot confirm that the transmission is in Park causing the vehicle to start in Neutral, but not to start in Park.
Symptoms of this problem is when the "P" indicator on the dash sometimes doesn't light up when the transmission in Park or you have to shift it back and forth a few times before the "P" indicator lights up. The shifter inside the transmission might also feel stiffer to move due to restricted movement of the shifter/cable/linkage.

The repair depends on the diagnosis. If there is minor rust at the cable/linkage, simply lubricating the affected area might help. In other cases, the rusted part might need to be replaced.
Lubricated pivot point at the shfter cable connectionLubricated pivot point at the shfter cable connection
For example, in this Mazda in the photo, a rusted lever at the transmission range switch has been replaced and the connection lubricated. It's a $20 part and a fairly simple repair. Another example: In some older Ford trucks with the column shifter, one of the brackets that holds the linkage under the dash can become loose and the truck would not start in Park - watch these videos describing the repair. The symptom is the same, the dash indicator is not lined up with the shifter position.

Sometimes, a malfunctioning or misadjusted transmission range switch can also cause the vehicle not to start in Park, but to start in Neutral. An electrical problem with the transmission range switch connector or wiring can also cause the same problem. The vehicle must be diagnosed properly to confirm this. A bad transmission range (position) switch must be replaced. It also needs to be properly adjusted after replacing. After the repair, it's a good idea that all gears shift properly and the indicator on the dash shows the correct position of the shifter.