Circuit Description

PCM controls engine idle with Idle Air Control (IAC) valve. To increase idle speed, PCM grounds OPEN coil windings inside IAC valve. A plate opens within IAC valve and allows air to by-pass throttle plate. To decrease engine idle speed, PCM grounds CLOSE coil windings inside IAC valve. PCM uses IAC valve to maintain a predetermined target idle speed to compensate for A/C compressor operation. DTC P0505 will set when idle speed continues to vary greatly from predetermined target idle speed. When DTC sets, MIL will illuminate after 2 consecutive ignition cycles in which diagnostic runs with active fault. PCM will record operating conditions at time of failure and store information in freeze frame buffer.