Idle Air Control Symptoms

Written by david mcguffin
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Idle Air Control Symptoms
Replacing the IAC valve is often a quick-fix that takes only a few minutes. (Jupiterimages/ Images)

There are a variety of sensors and valves that work together to help control the combustion and ignition process for automobile engines. When any one of these components fail to work properly, common symptoms include misfires and a loss of fuel efficiency. Knowing the symptoms of a bad idle air control valve can help you to troubleshoot and fix the problem quickly.

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Failed IAC Valve

The IAC valve helps to control the engine idle rpm through adjusting an air bypass inlet located within the throttle body. According to 2CarPros, there are two different ways the IAC can fail. In some cases, the motor for the IAC short-circuits and does not open and close the air bypass in the throttle body. In other cases, the motor malfunctions by developing more resistance than normal, allowing the car to idle too lowly and causing it to stall.


The IAC valve, also known as the IAC motor, is "highly susceptible" to carbon build-up, according to 2CarPros. Certain models are also equipped with a vacuum hose connecting the intake manifold to the IAC valve. If the vacuum hose is broken or has a small leak through normal wear and tear, then the engine will mimic symptoms of a bad IAC valve. It is also important to note that Toyota and Lexus models are equipped with a non-motorised, magnetic IAC valve. 2CarPros suggests cleaning the IAC air inlet every 40,000 miles to avoid stalling from carbon build-up.

OBD II (on-board diagnostics)

Plugging an on-board diagnostic II scanner to your car's OBD II terminal can help to diagnose a faulty IAC valve. However, it is important to note that an OBD II scan does not always show up on the OBD II scan. The OBD II terminal for your vehicle is typically located within a few feet of the steering column. Most manufacturers locate the OBD II terminal underneath the dashboard, between the base of the steering column and the driver's door, although the terminal is sometimes located inside or on the rear panel of the middle console.

Checking IAC Valve

Remove the IAC, which is located on the throttle body and plugs into the throttle body assembly. Leave the wires connected to the IAC valve and turn the ignition key to the "On" position. Do not start the engine. The IAC valve's motor is working correctly when it moves in or out when the ignition key is turned "On." Before replacing the IAC, 2CarPros recommends using carburettor cleaner to clean the IAC inlet passages on the throttle body.

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