Depending on just how much the transmission is overfilled, it may be no problem at all. Otherwise, the most common problem will be fluid leaking out the vent. Shift quality can be affected, especially if the fluid level is high enough to get aerated by the moving geartrain parts. This can cause damage if the transmission starts slipping and the fluid level is not corrected soon enough.

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Erich Loepke
2010 Ford Focus
Currently Benz-less