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Downloaded the entire section for the '06 Avalon automatic transmission. This vehicle uses the U151E tarnsmission.
Nowhere in this doc was there any indication that a diyer could not drain/refill his/her '06 Avalon automatic transmission.
I also posted this question to 5 area Toyota service depts. via their WEB sites. Here's the reply I received from Fred Driffill at Red McCombs Toyota in San Antonio, Tx. The photo that may not open is merely an excerpt from the '06 Avalon owners manual showing the refill quantity which is 3.7 qts.
There is no reason why a DIYER cannot change his/her own '06 Avalon automatic transmission fluid. Just be sure to use the Toyota WS fluid.
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The 2006 Avalon uses WS transmission fluid. Under no circumstances should any other fluid type be put in the transmission as it may cause erratic and hard shifting and possible damage.
That said, there is not any special procedure for draining and refilling the transaxle on the 2006 - 2007 Avalon. The confusion may be from some of our vehicles not having a dip stick (the common place for adding fluid if through the tube the dip stick is in). Our 4Runner and Tundra are a couple of our vehicles that do not have dip sticks and require special technical procedures to fill and check the fluid level.
When draining the transaxle you will only get about three quarts out of the pan while about four or five stay inside the transaxle and torque converter. To change all of the fluid you would need to have a flush performed. Toyota says that we must have a dedicated flush machine for WS fluid if we are to do flushes on vehicles that use WS fluid. As of right now we do not have a dedicated WS flush machine nor do we have any plans to offer this service in the foreseeable future.
We have recommended drain & fills every 30,000 miles on most of our vehicles. We recommend flushes when the fluid is excessively dirty. Typically it is at higher mileage and on vehicle that the fluid most likely has never been serviced.
Hope this helps and let me know if you need any additional information,
Frank A. Driffill
Parts & Service Director
Red McCombs Toyota
fdriffill@redmac.net
(210) 357-1667