Sealed means not consumer friendly. It is possible to DIY this. I have a 2014 Sienna with a “sealed” transmission and did the flush last month in my garage. The transmission has a fill and drain port. You just need to locate them. The drain will have a plastic standing pipe which sets the fluid level at a specific fluid temperature, normally 106F-115F. Once the fluid reaches that temp you top off the unit until fluid drains out and when it stops, you replace the drain plug. Its fairly easy except how to read the temp. I use an app called enginelink on my iphone with an OBD2 adapter called LeLink.
The flush is done by disconnecting the cooling line to the radiator. Turn on the engine long enough to pump out 2qts, then refill 2qts REPEAT until you have removed 12qts. The final fill is done when you are at 105* - 115*.
Drain plug: once you remove the drain plug, use a 5mm hex wrench and insert up drain, unscrew the plastic standing pipe and then drain out the trans fluid. Replace pipe and plug and add fluid. Flush trans. Via radiator hose. Top off by removing drain plug, allow trans to warm up, and add fluid until it drains out at temp. Replace drain and fill plugs. Done.
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