The Power Steering Pressure Switch (PSPS) opens as pressure increases. This sends a signal to the Engine Control Module (ECM) which adjust the idle speed to compensate for the additional load.
Disconnect the PSPS. With engine idling, measure resistance between the PSPS connector terminals. If the resistance changes from less than 10 ohms to infinity (indicating open PSPS) when the steering wheel is turned from center to more than one half turn left or right, the PSPS is okay.