Seven problems related to power train have been reported for the 2012 Toyota Sienna. The most recently reported issues are listed below. Please also check out the statistics and reliability analysis of the 2012 Toyota Sienna based on all problems reported for the 2012 Sienna.
Rear axle coupler assembly (part number 41303-28013) making very loud noise and vibration.
Vehicle began 'whirring' metal noise from underneath. Local mechanic said it was coming from rear differential. I took it to a Toyota dealer, they said rear differential coupler needs replacing ($2159) and drive shaft ($992). It seems the bearing ring inside this sealed unit has failed. Noise is continuous while driving, concern of part failure - we are taking vehicle on family vacation. Researching on - line it appears to be common problem with the Sienna's. Toyota had same issue with rav 4's and issued recall but have not for the Sienna's. I called Toyota corporate in TX (got up 3 levels) to no avail, and it curt tone, they said they cannot help. Based on other similar stories on line from other Sienna owners, this seems to be a common very expensive design flaw problem with this vehicle.
The rear viscous coupler on the awd had gone bad and needs replacement. This part is the same defective part on the rav 4 that had an extended warranty coverage due to bad design but is not covered on the Sienna. This part needs a recall.
The bearing for the viscous coupler has gone bad. It whines loudly when in motion, and the volume correlates with the speed. According to feedback from online this issue happens a lot. Toyota will not change the bearing ($50 plus labor). They will only change the whole coupler ($1800 plus labor) as they state the coupler is not serviceable. Multiple people have serviced the working coupler to change just the bearing. Toyota released a similar TSB for the rav 4 where the coupler failed.
When coming to a stop going about 5 mph towards a stop sign, my van accelerated into traffic. Because the van jolted me into oncoming traffic, it almost caused an accident. It doesn't seem to matter whether I am braking or coasting, the van will accelerate uncontrollably. I have two children and not only am I putting their lives at risk, but mine also. I am not able to drive this vehicle due to how dangerous it is.
While parked, driver began exiting vehicle and van began to roll and continued to roll when gear shift lever confirmed in park and brake continually pressed, eventually hitting a house.
All underwood components are not only vulnerable, but attractive to rodents. While parked overnight on 4 consecutive nights, rodents have chewed through wires despite attractive rodent poison in the engine bay, the poison is untouched and the wires are attacked every night. To date injector wires, sensor sires, fuel pump wires have been compromised, including sensors for the stability control system, which is disabled. I repaired damage by running new wires, clearing nests and set traps and poison, but now the the fuel system has been attacked and the fuel pump does not activate. 3 other cars parked in the same location for the past 5 months are untouched. Safety systems are likely to be compromised for any person who must park these outdoors.