Will over filling trans cause problems? Or will the extra fluid just come out the vent? Automatic

 

It really depends on how much it was overfilled by. My trans was overfilled by like 2 quarts by some shop and shifting started to become erratic and on the highway I would get a gearbox fault saying that the temperature in the box was too high. It would smell and squirt gearbox oil on the ground.

At this point I didn't know that the shop had overfilled my tranny and was thinking it may just be dying of age. I decided to give it one more shot by doing the fluid change myself. This is when I noticed it was overfilled. I corrected the problem with new fluid and my transmission is stronger than ever.