Transmission Fluid Pressure (TFP) Manual Valve Position Switch Assembly
WARNING: This page is about a different car, the 2007 GMC Sierra and 2007 Chevrolet Silverado. However, it is still accessible from the selected car via links, so may be relevant.
Lo fluid is routed to the TFP manual valve position switch where it closes the normally open lo pressure switch. The addition of the lo pressure switch being closed signals to the PCM that manual first is selected.