ECM controls engine idle speed using IAC valve. To increase idle speed, ECM retracts IAC valve pintle away from its seat, allowing more air to by-pass throttle bore. To decrease idle speed, ECM extends IAC pintle valve toward its seat, reducing by-pass airflow. A scan tester will read ECM commands to IAC valve in counts. The higher the counts, the more air is by-passed (higher idle). The lower the counts, the less air is by-passed (lower idle).

NOTE: Test numbers refer to numbers on diagnostic chart.

Tech 1 RPM control mode is used to extend and retract IAC valve. Valve should move smoothly within specified range. If idle speed is too low (less than 700 RPM), engine may stall. This may be normal and would not indicate a problem. Retracting IAC beyond its controlled range (greater than 1500 RPM) will cause a delay before RPM starts dropping. This is normal.
This test uses Tech 1 to command IAC to control idle speed. ECM issues commands to obtain recommended idle speed. Each node light should flash Red and Green to indicate a good circuit as ECM issues commands. Sequence of color is not important but if either light is OFF or does not flash, check circuits for faults, beginning with poor terminal contacts.

When to See a Mechanic

Stop DIY work and contact a certified mechanic immediately if any of the following apply:

• You smell fuel, burning insulation, or see smoke.

• Brakes feel soft, pull hard to one side, or make grinding noises.

• The engine overheats, stalls repeatedly, or misfires under load.

• You are missing required tools, torque specs, or safe lifting equipment.

• You are not confident in the next step or safety outcome.

Safety Warning

Chart C-2C Idle Air Control (IAC) System Check: Notes

Use the Manual With the Right Hardware

When to See a Mechanic