#5 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.
The applied shift solenoid valve - mode B closes off the PS5 circuit exhaust, allowing fluid pressure in the circuit to build to actuator feed limit fluid pressure. PS5 fluid opens the normally closed #5 pressure switch, signaling the TCM that the transmission is in Drive Range - Third Gear.