One problem related to transmission failure has been reported for the 2008 Toyota Sienna. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2008 Toyota Sienna based on all problems reported for the 2008 Sienna.
While driving on inter-stat I-10 west bound close to 183 marker, at 65 mph, the rpm shot to 7000 and car started to slow down. The car stopped with transmission failure. It was 2200 hours with my family of 4 kids and wife on board. To replace the transmission (used one 90000 miles) Toyota legacy charged me 3500$. Its a lot of money. Additionally, you cannot trust Toyota brand for such poor workmanship where power train is prone to failures on 5 years old vehicles.