Idle Air Control (IAC) System Diagnosis

Circuit Description
The powertrain control module (PCM) controls engine idle speed by adjusting the position Of the idle air control (IAC) motor pintle. The IAC is a bi-directional motor driven by 2 coils. The PCM pulses current to the IAC coils in steps (counts) in order to extend the IAC pintle into a passage in the Throttle body to decrease air flow. The PCM reverses the current pulses to retract the pintle, increasing the air flow. This method allows highly accurate control of the idle speed and quick response to changes in engine load.

Diagnostic Aids
Inspect for the following conditions:
^ A faulty connection at the PCM or the IAC motor. Inspect the harness connectors for backed out terminals, improper mating, broken locks, improperly formed or damaged terminals, and faulty terminal to wire connections.
^ Inspect the wiring harness for damage.
^ A restricted air intake system. Inspect for a possible collapsed air intake duct, restricted air filter element, or foreign objects blocking the air intake system.
^ Inspect the Throttle body for objects blocking the IAC passage or the Throttle bore, excessive deposits in the IAC passage and on the AC pintle, and excessive deposits in the Throttle bore and on the Throttle plate.
^ Inspect for a sticking Throttle plate. Also inspect the IAC passage for deposits or objects which do not allow the IAC pintle to fully extend.
^ Inspect for a condition that causes a vacuum leak, such as disconnected or damaged hoses, leaks at the EGR valve and EGR pipe to the intake manifold, leaks at the Throttle body, a faulty or incorrectly installed PCV valve, leaks at the intake manifold, a brake booster hose disconnected, etc.

Reviewing the Fail Records vehicle mileage since the diagnostic test last failed may help determine how often the condition that caused the DTC to be set occurs. This may assist in diagnosing the condition.

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
11. This vehicle is equipped with a PCM which utilizes an Electrically Erasable Programmable Read Only Memory (EEPROM). When the PCM is being replaced, the new PCM must be programmed.