Introduction to On Board Diagnostics (II)

Sensors and ActuatorsEmployed in OBD IIDiagnosticsIdle air control (IAC) valve Principle of operationThe valve is an electronicallycontrolled throttle bypass valve which allows air to flow around the throttle plate(which is closed due to low engine RPM and vehicle being stationary) and produces thesame effect as if the throttle is slightly opened. A stepper motor opens the pintle (valveallowing a limited amount of air to bypass the closed throttle plate. The steppermotor controls the pintle movement accurately thus controlling the amount ofbypass opening into the intake manifold. The duty cycle of the stepper motoris controlled by the PCM which monitors the pintle position and commands thestepper motor to move back the pintle to open the bypass by the calculatedamount and move the pintle forward to close the bypass at the end of the duty cycle.DiagnosticsOBD II DTCsThe duty cycle of the stepper motor , and the amount of bypass by thepintle valve are correlated periodically by OBD IIdiagnostics. The initial position and the final position of the pintlevalve are continuously checked.The failure modes of idle air flow are diagnosed by OBD II. DTCs for thesefaults are 505-507.