2003 neon 2.0 , automatic, 136,000 miles. Transmission went into limp mode. Took it to auto zone for codes. Said that I needed transmission shift module. Replaced the module. Changed the transmission fluid and filter. Old fluid looked very good, pink, no contamination.
After the work, problem was still there. After I hook up the battery, the car starts out in first, shifts to second, and locks into limp mode. Also, there is a new problem, the speedometer no longer works. Before the repairs, the speedometer worked fine. I have checked and rechecked the 4 connections on the wiring harness, soloniod plug, speed sensors, and transmission monitor plug. The plug to the transmission was a little loose. Also decided to change the 2 speed sensors. Nothing has helped. Unless I have somehow damaged the wiring harness, don't know why lost the speedometer. Would appreciate any ideas on what to check next.

 

My wifes 04 neon did the same thing and it turned out to be a broken wire on the output VSS inside the connector. one way to find out is to use an Ohm meter and put one end on the connector pin and then probe the wire 3-5 inches from the connector and observe the meter.