A bad transmission.
dirty tranny fluid
faulty speed sensor in the transmission
if it is automatic then this would be caused by shift down is not working in transmission
The manual shift transmission does not have a filter.
The gears in your transmission may be sticking or worn out. This would cause your transmission to not be able to shift.
The shift solenoids are inside the transmission, on the valve body.
Dodge has a transmission programming problem with ALL (manual also)2001 Dodge Ram Trucks. It had something to do with the speed to torq/ratio. Just tell them you need the program fix at your dealership. It should not cost you a thing. If you own a 5 speed manual with cruise control like me then be aware your shift light will not work any longer. It should also fix any lurching effect some trucks have when cold and you are trying to take off. Good luck!!!
Inside tranny
The transmission solenoid are inside the transmission on the valve body.
Allison transmission not shifting
Leaky transmission lines would cause the fluid level to drop and eventually cause the transmission not to shift and go.
The Dodge Stratus should solenoid can be found on the side of the transmission. The shift solenoid will be on the right side. The shift solenoid should be labeled as such.