If your vehicle is showing gear indicator (prndl) not showing the correct gear, here are the most likely causes and the parts you'll need to fix each one.
The plastic bushing connecting the shift cable to the transmission linkage degrades and breaks apart over time. This creates slack in the cable, causing the gear selector inside the cabin to be out of sync with the actual gear engaged at the transmission.
This sensor mounts on the side of the transmission and tells the vehicle's computer which gear is currently selected. If it fails internally, gets dirty, or suffers from water intrusion, the dash indicator will show the wrong gear, multiple gears, or go completely blank.
If the transmission, shift cable, or neutral safety switch was recently replaced or serviced, the cable may not have been adjusted correctly. This causes the indicator to be slightly off, such as highlighting Reverse when the shifter is physically in Park.
On older vehicles with mechanical linkages and individual bulbs for the PRNDL display, a burnt-out bulb can make it look like a specific gear isn't registering on the dash when selected.
The computer that controls the transmission can suffer from internal electrical faults or poor ground connections, causing it to lose communication with the gear selector sensor and display incorrect or erratic gear information.