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Hello and welcome to just answer! Jay, the most common cause of this "limp in mode" is a failing input or output speed sensor. There may be another sensor or solenoid circuit problem, but I usually see the speed sensor(s) failing and causing this problem. Do this: Have a scan tool that can read transmission codes and sensor data coneected to the vehicle and 1 see if there are transmission related codes, and 2 drive the vehicle while monitoring the input and output speed sensor signals. If one of the sensors is reading 0 rpm, then that is the one that will need to be replaced. If there are other transmission codes, then those codes will need to be diagnosed accordingly. Some auto parts stores may have the appropriate scan tool that can do this. Sometimes they will check it for no charge. Note: be sure that the scan tool can actually read sensor data for the transmission not just for the engine codes.
I hope this information helps, and let me know if you need further assistance with this.