One problem related to catalytic convertor 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.
2004 Toyota Sienna le--strong sulfur like odor emitting from exhaust especially noted during cold weather. Took vehicle in for evaluation and told no problem--most likely "quality of gasoline". A mechanic friend suggested "catalytic converter" problem. Concerned about long term consequences!.