If your vehicle is showing transmission slips out of gear while driving, here are the most likely causes and the parts you'll need to fix each one.
Low fluid levels reduce the hydraulic pressure needed to keep an automatic transmission engaged in gear. Old, burnt fluid can also lose its frictional properties, causing the internal clutches to slip.
In a manual transmission, the friction material on the clutch disc wears down over time. When it gets too thin, it can no longer grip the flywheel under load, causing the engine to rev without transferring power.
Shift solenoids control the flow of hydraulic fluid to change and hold gears in an automatic transmission. If a solenoid fails or gets clogged with debris, the transmission may unexpectedly drop into neutral.
Worn bushings in the shift linkage can prevent the transmission from fully engaging into gear. This makes it highly susceptible to physically popping out of gear when hitting a bump or accelerating.
If the transmission was recently serviced and underfilled, or if the wrong specification of fluid was used, the system will lack the proper hydraulic pressure to maintain gear engagement.
A broken transmission mount allows the entire transmission to twist excessively under acceleration. In manual cars, this twisting motion can physically pull the shift linkage and yank the car out of gear.
Severe internal wear, such as burnt clutch packs and bands in an automatic, or worn synchronizers and gear teeth in a manual, will prevent the transmission from holding a gear. This requires a major overhaul.