Complete Guide: What Happens When the Idle Air Control Valve Goes Bad?

The idle air control valve (IAC), also known as the idle air control actuator or auxiliary air valve, plays a significant role in the engine’s operation despite its small size. It is responsible for controlling airflow within the engine during idling.

So, what occurs when the idle air control valve malfunctions? 

Several issues may arise, such as engine stalling, abnormal idling, difficulty starting, a surge in engine speed, and decreased fuel efficiency. Read on for a more detailed explanation.

What Is the Idle Air Control Valve?

As you likely know, the engine comprises numerous parts that work cohesively to maintain proper functionality. 

The idle air control valve (IAC) is one such part, a tiny device located on the intake manifold attached to the throttle body. 

Its purpose is to regulate engine speed during idling.

Function of the IAC Valve

The core function of the IAC valve is to maintain a car’s idle speed. 

Idle speed, measured in revolutions per minute (RPM) and displayed on the car’s dashboard tachometer, is the speed at which the engine runs when the car is stationary.

The idle air control valve achieves this by controlling the amount of air bypassing the throttle plate. 

When the engine is idle, the throttle plate remains closed, preventing air from flowing into the engine and risking stalling or rough idling. 

The IAC mechanism prevents this by working with the engine control unit (ECU), opening to allow a limited amount of air to bypass the throttle and regulate idle RPMs.

Why Is Idle Speed Important?

The idle speed is vital because it influences the engine’s performance when subjected to extra load or certain temperatures. 

For instance, turning on the heater or air conditioner automatically increases the engine load.

Anytime you turn on these accessories, the engine control module signals the idle air control valve to open wider. This way, it allows more air to bypass the throttle so as to increase idle speed. 

A high engine idle speed is also necessary in cold temperatures, especially if you’re dealing with older cars. In such cases, increasing the amount of air flowing into the engine bay enables it to warm up faster. 

Usually, the normal idle speed of a passenger car ranges between 500 and 1000 RPMs. But under the situations mentioned above – cold temperatures and extra load – the car may register a high idle speed of up to 1,200 RPM before normalizing. 

Symptoms of a Bad IAC Valve

Like other engine parts, the idle air control valve is susceptible to failure. 

The two most common symptoms of a faulty IAC valve are a fluctuating idle speed and the illumination of the check engine light.

Fluctuating Speed

The first thing you’ll likely notice if the IAC valve is faulty is that the engine’s idle speed will start fluctuating.

If the idle speed does not remain constant or it keeps increasing and decreasing abruptly, there’s a high probability that the idle air control valve is damaged. 

Engine Warning Light

The idle air control valve’s operation is usually controlled by the engine control unit/module. If this 

system detects a problem with the IAC, it responds by triggering a warning light that pops up on your dashboard. At the same time, it keeps a diagnostic trouble code in its memory. 

Depending on the cause of the faulty IAC valve, these symptoms manifest differently. You may experience engine stalling or rough idling. 

Engine Stalling

Stalling is when a running engine comes to an immediate halt on its own. The engine relies on a specific air-fuel mixture to run optimally. With a bad idle air control valve, the appropriate amount of air doesn’t get delivered to the engine and this causes it to stall. 

Rough Idling

A rough idle means the engine doesn’t run as smoothly as it should when you put your car in park mode

Anytime you start your car, you expect the engine to fire up and provide a flawless operation.

But if you start experiencing an intermittent vibration or shudder and the RPM starts fluctuating, you’re experiencing a rough idle that’s likely caused by a bad idle air control valve. 

Causes of IAC Valve Failure

If you have a bad IAC valve on your hands, it’s likely being caused by one of these three issues:

Poor maintenance

If you have a tendency to use low-quality fuel or you rarely do maintenance checks, this can cause dirt and debris to accumulate on the IAC. 

Electrical Issues

As stated earlier, the system that controls the IAC’s function is the engine control unit. If there are any loose or broken connections within this system, they can also cause the idle air control valve to fail. 

Defective Engine Control Module

The engine control unit can also suffer damage, causing it to relay the wrong signals to the idle air control valve. 

Diagnostic Procedures

Now that you know the causes, what’s the easiest way to check for a bad air control valve? Well, most mechanics and car owners use one of these three methods to troubleshoot a bad IAC:

Using an OEM-level Scanner

The best tool to gauge the performance of an idle air control valve is an OEM-level scanner.

It offers identical functions to the dealer’s factory code scanner, making it extremely useful. While you can use an OBD-II or OBD-I scanner, it won’t have the same functions as a bidirectional scan tool for testing the IAC. 

OEM-level scanners make the process of examining IAC valves a breeze.

All you have to do is check the idle speed’s reading as you open and close the valve.

Ideally, the idle speed should increase as you open and decrease as you close. If this doesn’t happen, you’re likely dealing with a bad idle air control or loose circuit. 

Check for Error Codes

If you don’t have an OEM-level scanner, the next best solution is to check for error codes using an OBD scanner.

Simply connect the code scanner to the port underneath the dashboard.

Next, wait for the scanner to display the error code that’s causing the check engine light to illuminate. 

Cars manufactured after 1996 usually come with an OBD-II that will trigger a code for bad idle air control. Similarly, older cars (those manufactured before 1996) use OBD-I to trigger a code if the IAC is faulty. 

Here are some of the error codes that are likely to show up on the scanner and their respective implications:

  • P0505 – means the idle control system has failed
  • P0506 – means the RPM is significantly lower than it should be
  • P0507 – the idle air control valve’s RPM is higher than it should be

Check for Vacuum Leaks

If an error code shows up on the scanner, the first thing you should do is conduct a visual inspection of the circuit and vacuum lines.

This way, you can be sure that a vacuum leak or broken electrical connection isn’t the cause of the bad idle air control. 

Open the hood and check whether the vacuum lines are worn out, cracked, or have some other form of damage. A more surefire way of checking these lines is by connecting them to a vacuum gauge. This helps you determine the vacuum’s level and consistency.

If the vacuum is in pristine condition, inspect the IAC’s electrical connector next.

It should be clean and secured firmly in place. You’ll also want to check the wires that go into the idle air control valve.

None of them should be loose or broken at any point. If you notice any issue during your visual inspection, replace the damaged part and check for error codes again. 

Impact on Engine Performance

car engine

A bad idle air control can cause negative impacts on the engine’s performance. As mentioned earlier, one of the issues you’re likely to encounter is rough idling.

Other possible consequences are an engine that’s difficult to start, engine surge, and high fuel consumption

Difficulty Starting

The IAC is responsible for regulating the amount of air that enters the engine.

So if it’s faulty, it can cause the air-fuel mixture to be too lean. The end result of this is that the engine has trouble starting.

And in worst-case scenarios, it might not start at all; hence, affecting the car’s drivability.

Engine Surging

If the idle air control is stuck in the open position, then it will cause the engine to idle at a much higher RPM. As a result, the engine speed will surge or increase suddenly.  

Poor Fuel Efficiency

A defective idle air control can also cause the engine to consume more gasoline than usual. By failing to regulate the engine’s air intake, it causes an imbalance in the air-fuel mixture.

This means the engine will end up burning more fuel; compromising the car’s fuel efficiency. 

Potential Damage and Consequences

While it’s possible to drive with bad idle air control, I wouldn’t recommend doing so.

A faulty IAC can lead to multiple issues, such as difficult starts, rough engine idle performance, backfiring, and stalling. 

Some drivers think they can evade the problem by avoiding idling.

But here’s the thing: every time you stop at a stop sign or red light, your car idles. So you can’t really escape this problem unless you troubleshoot and fix it. 

Repair and Replacement

If you observe issues with your idle air control, consider cleaning it before rushing to replace it. Cleaning is easily achievable, as the IAC is readily accessible. 

Use a premium-quality degreaser, a clean cloth, and work gloves. If cleaning resolves the issue, the problem is likely solved.

However, if cleaning proves ineffective, a replacement may be necessary. 

IAC valve prices range between $50 and $100, with labor charges for installation varying between $70 and $300

Therefore, the total cost can range from a minimum of $120 to up to $400, depending on the installation complexity.

Preventive Maintenance

To prevent issues with the idle air control valve, regular cleaning is recommended. 

This ensures that carbon, dirt, and other particles do not accumulate and affect its functioning. 

While no specific guidelines exist for inspection frequency, checking its condition during each car servicing is a reasonable approach. 

Have your mechanic inspect all components associated with the IAC’s operation and replace or repair any faulty parts. 


Just what happens when the idle air control valve goes bad?

There are several problems you’re likely to encounter, including engine stalling, a rough idle performance, engine speed surge, and high fuel consumption. 


Dean Alvarez, TireForge Head Author

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