When a transmission slips out of gear, the engine is not adequately connecting to the crankshaft. Since the clutch is the piece of equipment that connects the engine to the crankshaft, a slipping transmission is almost always a clutch problem. If the transmission is slipping, the clutch is not doing its job correctly, and the engine is not as efficient as it could be.
Shift your car into first gear and drive slowly forward then shift through the gears while listening to the engine.2
Listen to hear if the engine is accelerating more than it needs to. This indicates a slipping gear -- the engine is accelerating and burning fuel, but it is not connected to the crankshaft so the wheels are not moving with it. If you hear the engine revving but don't experience an increase in speed, the gears are slipping.3
Change the transmission fluid. This is most likely the cause of transmission slippage if your vehicle has low mileage.4
Replace the transmission's internal filter. Over time, filters become less effective and will restrict fluid flow rather than filter it.5
Perform regular fluid and filter changes on your vehicle's transmission every 25,000 miles. This is a preventative measure; if you wait until the transmission slips, the damage may already be done and fixing it will require more parts and labor than a new filter and fluid.