A parking pawl is a device fitted to a motor vehicle's automatic transmission on order for it to lock up the transmission. It is engaged when the transmission shift lever selector is placed in the Park position, which is always the first position (topmost on a column shift, frontmost on a floor shift) in all cars sold in the United States since 1965 (when the order was standardised by the Society of Automotive Engineers (SAE)), and in most other vehicles worldwide.
The parking pawl locks the transmission's output shaft to the transmission casing by engaging a pawl (a pin) that engages in a notched wheel on the shaft, stopping it (and thus the driven wheels) from rotating.
Most vehicle manufacturers and auto mechanics do not recommend using the transmission's parking pawl as the sole means of securing a parked vehicle, instead recommending it should only be engaged after first applying the vehicle's parking brake. Constant use of only the parking pawl, especially when parking on a steep incline, means that driveline components, and transmission internals, are kept constantly under stress, and can cause wear and eventual failure of the parking pawl or transmission linkage. The pawl might also fail or break if the vehicle is pushed with sufficient force, if the parking brake is not firmly engaged. Replacement can be an expensive operation since it generally requires removing the transmission from the car.
It is highly inadvisable to use the parking pawl to stop a vehicle in motion. The pawl mechanism is not strong enough to stop a vehicle in motion or may not engage at all. Under that much stress, the pawl may break off in the transmission, leading to costly repairs.