Transmissions can slip for a variety of reasons.
A slipping transmission can indicate problems that could lead to the entire transmission failing. There are several causes for a slipping transmission, with some being more common than others.
Fluid
Low fluid levels can cause the transmission to shift poorly or slip. For an automatic transmission, the fluid provides the hydraulic pressure and lubrication essential for it to work properly. Checking your transmission fluid and refilling as needed is one way to eliminate this problem. Note the color; dark or brown indicates overheating. Also check to make sure that fluid is not leaking; leaking fluid would be a reddish color, typically under the car.
Transmission Bands
Transmission bands are what links the transmission gears together. If the bands are worn down or broken, the transmission will start to slip and not shift gears properly. Worn bands can also cause other damage, including disabling the transmission completely.
Worn Gears
Just as worn bands can cause slippage, worn gears within the transmission can also cause the transmission to slip. This is because the gears do not link properly when they are worn or broken.
