Question

2003 Dodge Neon won't shift out of first, why? Replaced ECM (PCM)? Reads engine codes P0700 and P0508

Answer 1

P0700 Transmission control system ,fault detected, MIL light requested on

P0508 IAC (Idle air control valve) sense circuit low

Do you have the other transmission code? This can be a incompatible PCM, when the sense circuit is low to the IAC the sense circuit is low to the speed sensor.

Answer 2

P0508 Dodge Code - Idle Air Control Valve Sense Circuit Low
Read more: https://www.autocodes.com/p0508_dodge.html

P0700 Dodge Code - Transaxle Control System Malfunction
Read more: https://www.autocodes.com/p0700_dodge.html