Idle Air Control Valve – Symptoms of a Bad or Failling and How to Check?

Air is needed when oxidizing the fuel to produce mechanical energy. This energy is what makes the car run.

An idle air control valve is one of the most important components of the car's engine that controls the air being used for the engine.

But certainly, there are times that it will fail due to a variety of reasons. You must know then how to check the idle air control valve for failing symptoms and what to do when it becomes faulty.

Functions of the Idle Air Control Valve

Idle speed is identified as the RPM of the engine speed while the car is at a stop. This idle speed should remain the same so that your car can be ready to go when you press your foot on the throttle.

Since there's no gasoline needed to maintain the idle speed, the computer (through the valve) must input a minimum number and maintain it constantly while the car is at a stop.

An idle air control valve's function is to control and maintain the idle speed of the engine. This also helps in starting the car when the engine is cold. The valve can regulate the idle speed either increasing or decreasing it to match the operating conditions.

The shortened term for this component is IAC valve. It controls the idle speed by letting air in controlled proportions.

The concept here is to maintain a 14:1 ratio of both air and the fuel. Air is a difficult matter to control in essence as it’s an unstable component in which the density changes along with the atmospheric pressure and thermal heat.

The parameters in adjusting the idle speed are the engine temperature and electrical system load.

The idle speed regulation is also monitored by the coolant sensor, brake switch, and vehicle speed sensor. These are additional sensors that the computer uses to make the idle speed constant depending on the operating conditions. 

Symptoms of a Faulty Idle Air Control Valve

The biggest enemy of the IAC valve is the channel blockage. This results from the excessive carbon buildup.

This can also result from the particulates and contaminants that stick to the idle air control channel while the engine is at an idle.

These particulates bypass the air filter and mix with the oil. This, in turn, blackens the oil and you get a nasty and oily buildup.

Furthermore, this buildup gradually reduces the air flow until the valve can't get air anymore.
The list below answers the question as to how to check the idle air control valve for failing symptoms. These symptoms can only be observed, of course, when the engine is idling.

1. Engine stalling

An IAC valve that is close to failing may exhibit stalling symptoms. This means that as soon as the car is started, the engine stalls almost immediately.

Furthermore, engine stalling may occur while operating is a big pain. In another case, as soon as the car drives to a stop, the engine dies away immediately.

The engine stalling is caused by the IAC valve not getting enough air to burn the fuel to become energy.

This is exhibited when the IAC valve is just clogged. However, if it's not due to clogging, a symptom like this becomes a severe case.

Depending on the diagnosis of the mechanic or someone with the automotive background, you might have to replace the idle air control valve.

2. Irregular idle speed

A problematic idle air control valve has irregular speed. This is easily determined at the speedometer where the idle speed is unusually high or unusually low.

If not, the idle speed may arbitrarily change while at a stop. This is not normal because it is programmed that the idle speed of the engine must be at a constant rate. High idle speeds can cause engine overheating.

3. 'Check Engine Light' illuminates

When this illuminates on your dashboard, it only means that something is interfering with the idle air control valve circuit or its signal.

This signals you, the driver, that it must be checked thoroughly. Though there may be other cases why it illuminates, the IAC valve issue is the most common.

How to Check an Idle Air Control Valve

Another name for the idle air control valve is idle speed control valve. The IAC or ISC valve is located near the vehicle's intake manifold.

It is a motor controlled by an engine control module. When it fails, it causes a lot of issues on the car. The worst case scenario is it could render the car useless.

1. Check for vacuum leaks

When the engine idle speed is unusual or the engine is stalling, the first step is to check for vacuum leaks. If it’s proven that there’s a vacuum leak, the idle air control valve isn’t at fault.

If there is a vacuum leak, it means that there's an air leak at the engine bay. Then, to compensate, the engine computer decreases the idle speed through closing the idle air bypass circuit.

The difference between a vacuum leak and a bad IAC valve is simple. While the vacuum leak can suck air to oxidize with the fuel, the air leaks before reaching the engine. On the other hand, an IAC valve may suck little to no air at all. Also, there are no signs of leaks.

The vacuum leak can be identified by finding the IAC valve in closed position. The vacuum lines that encompass the valve area may have cracks or worn areas. These signs of damage are replaceable with new vacuum lines.

But before handing your car to the mechanic, remove the vacuum lines and connect it to the vacuum gauge. Then, measure the vacuum level and its consistency. If they're still good to go, they can be repaired and not replaced.

2. Check the idle speed

Most vehicles have an average of 1000 RPM. A problematic idle air control valve will make the idle speed unusually higher than the normal idle. Also, as said previously, vacuum leaks can cause high or low idle speeds.

The IAC valve can also make the idle speed way lower than 1000 RPM which is not good. Compare the current idle speed and the normal idle speed by numerous trials before concluding that it's a bad IAC valve.

3. Use code scanner

If there are issues like bad IAC valve, the check engine light will turn on. You must plug in a code scanner tool into the diagnostic connector.

For car models after 1996, you should use an OBD II scanner. The scanner will then summon the error codes that identify which made the check engine light illuminate.

These code scanners are easy to read because there's a provided English description for each error code.

4. Disconnect the idle air control valve

One of the ways to check if the IAC valve is functional is to disconnect it from the engine. First, locate the IAC valve. The location varies depending on the car model and make.

Then, once located, disconnect the IAC valve from its electrical connector. Make sure to unplug the IAC valve before starting the engine.

Next, start the engine to see how it will react. With the disabled IAC valve, the bypass circuit may open again and correct the stalling of the engine.

If things are now in favor, turn off the engine and let it cool down. Reconnect the IAC valve to the electrical connector. Then, start the engine again. Engine idling will turn back to normal.

5. Cleaning the idle air control valve

Just like the previous method, locate the IAC valve and disconnect it. Remove it fully by unscrewing the bolts or screws that attach it to the engine.

Remove the wire assembly too but carefully. Remember which of the wires are for current, ground, and computer.

A problematic IAC valve stems from the buildup of carbon, dust, etc. You will have to clean the IAC valve. Use a carbon cleaner or intake cleaner solvent to clean the IAC valve.

If you don't have them, you may also use carburetor cleaner. Then, with a rug, remove all the dirty buildup until there are no deposits left. Focus more on the spindle and pintle areas. A cloth rug will do because a metal brush can damage the IAC valve.

Install it back again by connecting the wires to their original places and screwing the bolts. Then, start the engine to see if the idling returns to normal. If you used solvent, it takes some time for the engine to burn it off though.

6. Check the idle air control valve's resistance

This method requires that you remove the IAC valve from the vehicle completely. The removal of the IAC valve is the same as the previous method.

With a multimeter or voltmeter, you will check the resistance of the valve to see if it's still functional or not. It the reading is within the specification, you will have to replace the valve with a new one.

The recommended resistance of the valve should be within 0.0 to 0.05. Set the multimeter to read ohms (the unit for resistance) and insert the terminal pins at the valve's electrical connector openings.


There are a number of methods on how to check the idle air control valve for faulty symptoms.

Once you have identified the symptom, check the valve through the above methods. If it's cleanable, then it's the best case scenario.

If it needs replacing, you may have to bring it to the automotive shop. New IAC valve may range from $150 to $350; excludinglabor costs.

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