1996 Avenger V6 Auto / Tranny will shift out of 1st when very cold to 2nd only - 1 time - after that it stays in 1st.
Sensor or TCM?

 

Does the speedo Read ?
Sounds like you are in limp mode

Could be input or output speed sensor , if the speedo reads then it could be the input sensor , if the tranny does not read its the output sensor

You can get the trannt scanned for codes

 

Speedo works fine - no check engine light on either?

 

the tranny will not trip a check engine light but a high order scanner can read the tranny codes

 

input or output sensor...or (3or4)relays on firewall dont let them tell ya its the tranny...