So my younger sisters 2001 Neon (auto) is have not running, the trans will not go into any gear just reverse. She told me the car was shaking/jerking going from a stop to 30mph and then it would go away and drive fine, but now the trany will not go into any gear just reverse. I checked the codes and PO734, PO1899 and PO551 any help would be great thanks.

 

P0734 - transmission related problem
I'm going to guess it's the output speed sensor (Also know as vehicle speed sensor) on your trans isn't working.

It's a cheapez sensor. I replaced both my input & output sensors for $11-$15.

 

If you want I can unplug the VSS connector on my trans, and then test drive my car to see how it shift.
But you have to wait until Monday for me to do this, because I parked my car in the garage. I don't want to wake up my family.

I know when the VSS is bad: your car will not shift, shift at the wrong time, shift up, and down all the time.
But I don't know if it will shift into 1st at all (on Neon cars).

p0734 is 4th gear wrong ratio
It can cause by a faulty VSS

 

Look on your speedometer when the VSS is bad, the speed would jump everywhere.