When a car won't shift into reverse or drive, there is usually a problem with the transmission, such as low transmission fluid or loose throttle cables. Transmission fluid lubricates the gears and other components of the transmission. When the car is low on this fluid, it cannot switch gears into reverse or drive.
Some vehicles with automatic transmissions require transmission fluid changes after 60,000 miles, while others never need fluid replacement. Manual transmissions require more frequent changes, such as every 30,000 miles. However, it all depends on the make and model of the vehicle and how frequently it is driven.
When a car won't shift into reverse or drive, the driver should open the hood to check the transmission fluid level. The precise procedure varies from one model to another, so it is necessary to consult the vehicle's manual for accurate instructions. When using a dipstick, it is important to look for continuous fluid coverage. If there are any gaps below fluid on the stick, the actual reading is at the gap.
A loose throttle cable can also cause difficulty shifting into reverse or drive. This braided cable is located under the hood. The nuts that connect the cable to the vehicle can be adjusted with a lug wrench, but this should only be attempted by those who are experienced at working on cars.