2-3 Shift Solenoid (SS) Valve
WARNING: This page is about a different car, the 2009 Cadillac DTS. However, it is still accessible from the selected car via links, so may be relevant.
The 2-3 SS valve de-energizes (turns OFF) in order to allow fluid from the 2-3 SS valve to exhaust through the solenoid. In the absence of 2-3 SS valve fluid pressure at the 2-3 shift valve, spring force acting on the end of the valve moves the valve into the downshifted position.