For cars 75,000 miles or over, the likelihood of a crack forming in the seals around your transmission is more likely. After the cracks form, a slow leakage of the transmission fluid (red or brown) will run onto the ground. This eventually will cause transmission malfunction thus vehicle malfunction. This How-to gives an overview of the options to fix these leaks.
Determine if you have a leak. The most obvious sign to a leak is a puddle of liquid on the ground. Park your car on a level surface after running the engine until warm. If you have a leak, the transmission fluid will drop onto the ground from either the front of the transmission or the back (the respective location of the seals). The transmission is generally located towards the front center of the car. If you stand by the driver's side door and peer underneath the car, you will likely see the leak on the ground. The fluid is either a brown or red color, depending on your make and model (consult your owner's manual).
Determine the seriousness of the leak. The amount of fluid lost will determine the action you must take.
*For minor leaks (a few drops in one area): This can be remedied quite easily. Go to Step 3.
*For major leaks (more than a few drops in one area): You may try a bottle of stop-leak, but inevitably you will need to take your transmission in for service at a dealership or transmission repair/service center.
For minor leaks Add stop-leak. Purchase a high-quality brand of transmission stop-leak (appropriate for your make and model). To add the contents, locate your transmission fluid dipstick (probably red) under the hood of your car. Pour the stop-leak directly down this tube. The viscosity of the stop-leak will be very thick, like molasses. Pour in just enough to replace the lost fluid. Use the dipstick to determine the correct level.
