Circuit Description
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 DTC P0505 Idle Air Control (IAC) System diagnostic observes engine speed to monitor operation of the IAC valve. Powertrain Control Module (PCM) controls engine idle speed with IAC valve. To increase idle speed, PCM pulse width modulates the OPEN solenoid within the IAC valve. This action turns a cylinder within IAC valve, allowing air to bypass throttle plate. PCM regulates fast idle speed via the IAC valve by increasing the opening of air bypass port. PCM regulates slow idle speed via the IAC valve by decreasing the opening of air bypass port.