What does it mean?
While driving, your foot on the accelerator pedal effects a change in throttle opening, which, in turn, effects a change in engine power and speed. When you release the accelerator pedal, the throttle closes, returning the engine to idle speed. To maintain engine idle speed, the IAC (idle air control) valve allows a small amount of air to bypass the throttle valve. Some manufacturers put the IAC in the throttle body, while others may place it elsewhere.
Many modern vehicles, drive-by-wire vehicles with an electronically-controlled throttle body, do not employ a separate IAC, but control idle speed by varying throttle opening directly. The ECU (engine control unit) modulates IAC valve function to control idle speed, using the CKP (crankshaft position) sensor for feedback. At any particular IAC value, whether opening percentage or duty cycle, the ECU expects the engine to be running at a specified rpm.
The ECU also modulates IAC to change idle speed, depending on engine load, such as high-idle when the air conditioning compressor is activated or the power steering pump is active. If the ECU detects a problem in the system, such as engine idle speed being higher or lower than expected for the IAC opening, it will set a DTC (diagnostic trouble code) and illuminate the MIL or CEL (malfunction indicator lamp or check-engine light).
If engine rpm is higher than expected, by 200 rpm or more, the ECU will set DTC P0507 – Idle Control System RPM Higher than Expected in memory.