Your transmission shift solenoids mount on the transmission valve body that controls fluid flow through the various circuits and passages in the transmission. The solenoid plungers are constantly bathed in fluid from the transmission, and metal salt from the manufacturing process and normal wear, as well as detritus from the friction materials can collect on the plungers and cause them to bind in their bores and fail to function properly. Identify these failures by the symptoms they produce.
Your transmission relies upon the positions of multiple solenoids to control the gear selection. If one or more solenoids are bad, you may lose the use of one or more gears, and may even be stuck in one particular gear or unable to shift into any gear at all. Do not confuse a slipping transmission with a solenoid problem. You will hear and feel the difference when you try to shift. A slipping transmission will actually shift, but then not produce any power once in gear, whereas a bad solenoid will prevent the shift from occurring in the first place.