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Hello. Welcome to Just Answer. Please allow me to assist you. This sounds like you have a vehicle speed sensor issue. This unit is responsible for transmitting spee mph to the speedometer, and, this same information controls the shifts of the transmission. This sounds like the sensor has failed and the car is in what we call Limp mode. The recommendation I jhave for you on this request would be to have the vehicle attached to a compatible scanner and the actual readout of the sensor determined. If the read out data is zero, either the vehicle speed sensor needs replaced, or, there may be damage to the wires on the sensor. If the vehicle was driven in high water, there may be moisture inside the wire connector that will cause a reduction in the output of the sensor to the wire harness