If your vehicle is showing car won't go into reverse, here are the most likely causes and the parts you'll need to fix each one.
Low fluid levels or a clogged filter prevent the transmission pump from generating enough hydraulic pressure to engage the reverse gear. This is the most common cause in automatic transmissions.
The physical cable or linkage connecting your shifter to the transmission relies on plastic bushings that degrade over time. When they break, moving the shifter doesn't actually move the lever on the transmission.
If the clutch doesn't fully disengage due to a hydraulic leak or a worn friction disc, the transmission input shaft keeps spinning. Because reverse gear usually lacks a synchronizer, this makes it nearly impossible to shift into reverse without grinding.
In automatic transmissions, electronic solenoids direct fluid to change gears. A failed reverse solenoid or a clogged valve body will prevent the reverse gear from engaging, even if fluid levels are fine.
Also known as the transmission range sensor, this tells the computer what gear you have selected. If it fails, or is misaligned after recent transmission work, the vehicle won't recognize that you've shifted into reverse.
Catastrophic internal wear, such as a broken reverse band, damaged planetary gears, or blown internal seals, will cause a complete loss of reverse. This requires a major overhaul.