One problem related to transmission slip out of gear has been reported for the 2004 Toyota Sienna. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2004 Toyota Sienna based on all problems reported for the 2004 Sienna.
Once, while driving on I-5, on a slight incline in 'd'(overdrive), I slowed the vehicle from 60 mph to around 50 mph because of traffic. The vehicle then seem to slip out of gear, the rpms hit approx. 6000, then the vehicle caught a gear. It repeated this several times. I pulled over and call the dealer. He said to if there were no warning lights, then it's probably ok. The second time we were in 'd' and slowing from about 60mph to 50mph coming into a slight corner. The vehicle repeated the same jerking. I notice that when slowing the rpms dip below 2000-1800 before the slipping occurs. I kept the car above 2000 rpms at higher spends by shifting into '3' when needed. This seem to avoid the problem? we brought the car in to the dealer. I was told what happen is normal operation for the new transmissions in the Sienna and highlander? however, losing power and being jerked around did not seem safe. I am bringing it to another dealer on this coming Friday.