#3 Pressure Switch
WARNING: This page is about a different car, the 2008 GMC Yukon and 2008 Chevrolet Tahoe. However, it is still accessible from the selected car via links, so may be relevant.
Solenoid 1 signal fluid passes through the hybrid direct, 2-3-4 clutch regulator valve into the PS3 fluid passage. PS3 fluid opens the normally closed #3 pressure switch, signaling the TCM that the transmission is in Drive Range - First Gear.