On fuel injection engine, idle speed is controlled by PCM/ECU or preset by the manufacture. However, you can adjust the idle speed by turning the adjusting screw in/out slightly. Since you alter the idle speed preset by the ECU, sometimes you will get engine code on indicating idle is lower/higher than normanl speed. It's not recommended practice!! If your car can only maintain proper idle speed by altering idle speed manually,there must be something wrong.
You can alter idle speed on arburator engine by turning the adjusting screw in/out slightly.