The B2 piston/band controls the 1st gear. When the piston and sleeve is bad, it starts to bind. With so much hydraulic pressure on the piston, in time, the piston can get stuck and no forward gear will work.
You don't need to remove the valve body. It's located behind a round cover on the right side of the trans.
Here is a write up about how to replace it.
b2piston
B2 piston replacement
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