The automatic transmission fluid in your Nissan Pathfinder should be changed in accordance with the manufacturer's recommended maintenance schedule. The transmission fluid is used to pressurise portions of the transmission, driven by a pump inside the transmission. The fluid pressure forces the transmission to shift and respond properly; if the fluid is not maintained, it can become thin and less effective, causing the transmission to shift erratically or cease to function at all.
Position a drain pan under the transmission oil pan just below the drain plug. You will see the plug in the flat pan on the bottom of your transmission.
Remove the drain plug with a wrench or socket and ratchet, being careful not to drop the plug into the drain pan. Allow the old transmission fluid to drain into the pan until there is no more coming from the drain.
Reinstall the drain plug and gasket, tightening it with a socket and ratchet until it is snug. Do not over-tighten the plug, or you will strip the head or cause the plug to become seized into the drain. Wipe the pan down with a clean rag to remove any fluid that may have run down it.
Remove the dipstick from the dipstick tube and insert a clean funnel. Slowly fill the transmission with new automatic transmission fluid. Start with three quarts, then check the level with the dipstick.
Continue adding automatic transmission fluid until the transmission is full. Use the dipstick to determine when it is full. Replace the dipstick and dispose of the used transmission fluid appropriately.
Most auto parts stores are now accepting used oils to be recycled. Check with a store in your area to see if they can take your used transmission fluid or check with the city or town hall for proper disposal procedures.