A malfunctioning transmission valve body will prevent your vehicle from changing gears as the operator desires. Each of the shifts from one gear to another is controlled in this mechanism via a hydraulic means. They all work automatically when the correct level of pressure is present as indicated by the transmission control device. The inner components of the transmission valve body can malfunction when the hydraulic fluid is contaminated with debris or become burnt from the slipping of the clutches in this component. This will prevent the valve from opening properly so the hydraulic fluid cannot flow which is what shifts the gears to power the vehicle.
The transmission valve body is located on the bottom of the automatic transmission and has many pressure valves located in it. Each shift from one gear to another has a separate valve in this component. The valves in the transmission valve body are made with springs, a valve stem and seals on the end so the pressure from the hydraulic fluid can be used to open and close them. On newer models the valves are controlled by the onboard computer that sends a signal when they should open as dictated by the speed of the vehicle and the hydraulic pressure in the transmission.
To replace the transmission valve body, the transmission fluid has to be drained and the transmission bottom pan has to be removed. The filter should then be pulled exposing the bottom of the transmission valve body. When this component is removed, the technician must be careful not to lose any of the check balls in it or the transmission will not be able to shift gears when it is put back in place.