Power Steering Pressure Switch
WARNING: This page is about a different car, the 2003 Pontiac Vibe. However, it is still accessible from the selected car via links, so may be relevant.
The Power Steering Pressure (PSP) switch is normally open to ground. Turning the steering wheel increases the power steering oil pressure. Increased power steering oil pressure puts an additional load on the engine that may be noticeable at idle. The PSP switch closes when the power steering system oil pressure is high enough to cause an undesirable idle condition. The voltage signal from the PSP switch to the PCM indicates less than one volt when the PSP switch is closed. The voltage signal is near battery voltage when the PSP switch is open.