My toyota sienna 2002 turn on and goes in gear but when i accelerate forward it stalls

sold new to what country? ( I will answer USA answers only)
which engines is there,? Gen1 car. 2005
answer
3.0 L 1MZ-FE V6 (gasoline)

down shifts are normal on all A/T boxes, if 2 things happen(either or both)
up hills
or passing (means throttle pedal push to floor, right foot)
tell more, driving modes are complex and for sure the gear box.
it runs this box below>
Transmission 4-speed A540E/A541E automatic
are you saying the the tranny ,FLARES , or false neutral?
2 tn 3 gear shift fails and flares (huge RPM increase is that)
if yes, that is transmission problem.!

so scan the PCM see why? if lucky it tells you something easy to replace. but flare is not a good sign. ever.

solved:
beg,barrow,rent or buy the scan tool or get it scanned in any real Car repair shop, (they know what to do , so let them)

Have you checked the transmission fluid?

The Flasher is mounted high on the Firewall behind the dash. You need to get on your back on the driver's side floor to view and reach the relay.
Before replacing the flasher, check the turn signal fuse.
I hope this answer will help. Goodluck.

Not an easy fix, plastic gears either wear out or get clogged over the years with dirt and lint. Whole motor assembly and track cost around $300 new.

Might be torque converter. Is engine light on?

Check the idle air control

Thanks for choosing FixYa and welcome to the site. There are a few different things that can cause stalling. It could be a clogged fuel filter or air filter. If you haven't had a tune up in a long time, it may be your plugs, coils, and wires. If you have a check engine light on, take it to Autozone or Advanced to have it scanned, They do it for free and it takes a minute. Your computer may have detected the problem. They can tell you what is wrong. Check for any vacuum leaks or loose hoses and wires. Check all your fuses. If it is one of your sensors, the computer scan should show it.

My 2002 Sienna just started doing this exact same thing. Curious if you ever found out what was causing this to happen?

Changing the fluid will never harm your trans unless you do not fill it back up.

Reverse is all hydrolic. There is no electrical that could cause it. If you have park and forward, you don't have a lincage problem.

As much as I feel for you, (I satarted rebuilding transmissions in the 80's) you will have to get it rebuilt.

Sorry

You have to "Teach" the tire sensors to the correct pressure. The 02 Sienna does not measure the actual PSI tire pressures, it measures the RPM of the tires. with the tires set to specs, the rotation RPM is "x". If a tire goes low, the rotation RPM would bt "y", and that will light the light.

The proceedure: Set the tires to specs with a known good pressure gauge. The ones that you can put in your pocket, that looks like a pen, are alway sincorrect. Get one with a needle reading. Set the tires correctly. Now, press the reset button, hold it until the light blinks 3 times. Drop it into gear, and DRIVE IT. 8-12 miles, above 19 MPH, and UNDER 61 MPH. This will re-teach the ABS computer the correct values.