I did not come up with these instructions on my own. I had programmed keys for other cars pretty easily, and was frustrated the minivan was so difficult to program. I had a locksmith scheduled to come out today, but after he called to reschedule, I found these instructions online. Happy to say it worked.
If you're looking for instructions on how to program a key fob, look here: http://www.ehow.com/how_6128421_program-keyless-en...
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Step 1: Buy a Transponder Key From the Internet
The Toyota Sienna uses a 4D-67 key. I bought two for $15 on ebay. These are pretty widely sold. Check the seller's reputation, but they are cheap enough that there isn't much sense for them to be selling fakes.
Step 2: Get You Key Cut by a Locksmith
I took my keys to ACE. They charged me more than most locksmiths, and they did a crappy job. A bunch of the keys I had cut (did more than just for this minivan) didn't work. I'm going to a locksmith next time.
Since they aren't selling the key, they are likely not going to promise the key will work, but locksmiths will typically duplicate keys cheaply.
Step 3: Follow These Steps to Program Your Key
Sit in your van with all doors closed. I opened and closed the door one time to 'clear' the system.
Insert your original master key to key cylinder 5 times and leave it there ('in' and 'out' four times, just 'in' the fifth time).
Open and close driver side door 6 times. Should be closed when you do the next step.
Remove master key from key cylinder, insert your new cut transponder key.
Turn that new key to "ON" position.
Look at that red engine immobilizer light (the red light to the left of the speedometer which is car shaped with a black key in the middle of it). It is blinking.. wait about 1 min and it will turn off by itself.
After engine immobilizer light turns off, remove new key and insert original master key. Start engine.
Now try your DIYed new transponder key to start your Sienna.