Transmission Fluid Pressure Switch
WARNING: This page is about a different car, the 2003 GMC Savana and 2003 Chevrolet Chevy Express. However, it is still accessible from the selected car via links, so may be relevant.
The Transmission Fluid Pressure (TFP) switch is actually 5 pressure switches combined into a single unit mounted on transmission valve body. The PCM supplies battery voltage on 3 separate wires to TFP switch. As switches are actuated in various combinations during transmission operation, PCM can detect what gear transmission is in.