2-3 Shift Solenoid (SS) Valve (315B)
WARNING: This page is about a different car, the 2008 Buick LaCrosse and 2008 Buick Allure. However, it is still accessible from the selected car via links, so may be relevant.
Energized (ON) by the PCM, high pressure in the 2-3 signal fluid circuit holds the 2-3 and 3-4 shift valves in the downshifted position.