1999 sld automatic

Am working on transmission .cause it wont shift into 4 gear. All other up shifts and down shifts fine. Have changed the following speed sensor ,egr valve ,coolant sensor .engine and transmission adjusted linkage changed fluid ,filter cleaned all connectors . On the right side of bell housing is the speed sensor . But on the left side below the filter and coolant fluid sensor there is a sensor that is plastic. That looks like another speed sensor . What is this? Any help would be a blessing . Also changed and used a volt meter to set the park nuetral safety switch SORRY ITS A 99SL/2

Did you check the connector on the ECTS?

Its possible your valve body is having issues. miles on the car?

The sensor near the filter is the turbine speed sensor.

did you check the fuses?