How to Check an Idle Air Control Valve

Commonly called an idle air control (IAC) valve or idle speed control (ISC) valve, this device is used on many cable-operated throttle bodies to regulate the engine idle speed. This process is completed by regulating airflow through a bypass circuit around the throttle plate to increase or decrease idle speed. Increasing the volume of air that flows through the bypass circuit around the throttle plate increases idle speed. Reducing the bypass airflow decreases idle speed. The engine control module controls and monitors this process and can command the fuel mixture to compensate for the airflow.

When we refer to the engine idle speed, this is the speed in revolutions of the engine when the driver's foot is not activating the throttle. While the throttle plate is closed, the main intake passage for the engine is closed off so a bypass circuit is necessary to keep from choking the engine.

When the engine's idle speed is above or below the predetermined range in the computer's program, the computer commands the valve to either increase or decrease the bypass air flow. Additional sensor inputs from the coolant sensor, brake switch and vehicle speed sensor may also be used by the computer to regulate idle speed according to various operating conditions. Idle speed may also be increased when the A/C compressor is engaged, the alternator is charging above a certain voltage, and/or the automatic transmission is in gear to prevent the engine from lugging down.

Part 1 of 4: Identifying idle speed problems

  • Tip: If the engine speed is too high, too low, or stalling, the problem may not be the idle speed control system, but an engine vacuum leak. Check for vacuum leaks first to rule out this possibility.

  • Considering a vacuum leak, a common condition is to find the IAC valve extended all the way out (closed position). This usually means the engine has an air leak and the engine computer is trying to bring the idle speed back down by closing the idle air bypass circuit. If there is an open or shorted circuit in the idle air control solenoid, driver circuit, or the idle speed is out of range, it will usually set one or more fault codes and turn on the Check Engine light. If the light is on, you must plug a scan tool into the diagnostic connector and read out the codes that set the light.

  • Note: Many modern vehicles are equipped with a “drive-by-wire” induction system. These vehicles use an angle sensor on the throttle pedal to calculate throttle input. The engine computer will then activate a small motor on the throttle body which also has a position sensor so the computer can match the throttle pedal angle to the throttle plate angle.

  • “Drive-by-wire” vehicles typically do not have an idle air control valve because the computer will take information from all of the sensors and automatically adjust the throttle plate angle as necessary. An idle malfunction here may require cleaning or the replacement of the throttle body as well as the use of a professional scanner to reset the system.

Materials Needed

  • Digital Multimeter
  • Professional Vehicle Scanner
  • Q-Tips or Pipe Cleaners
  • Throttle Body/Intake Cleaner
  • Vehicle Service Manual

  • Warning: The difference between throttle body cleaner and brake cleaner is the throttle cleaner does include a bit of lubricant for the moving components of the throttle body. DO NOT use throttle body cleaner on brake system hardware.

Part 2 of 4: Checking the IAC function by disabling it

Step 1: Gain access to the IAC valve. Consult the vehicle service manual for the location of the IAC valve on your vehicle.

Step 2: Disconnect IAC valve. Locate the IAC valve electrical connector and unplug the IAC valve.

Step 3: Start the engine. Start the engine and observe how the vehicle reacts. Where a vehicle may have previously been stalling after start-up, disabling the IAC valve my open the bypass circuit and cause the vehicle idle to raise when the valve is disabled.

Step 4: Reconnect the IAC valve. Turn off the ignition and reconnect the IAC valve electrical connector.

Step 5: Start the engine. At this point the engine idle should return to normal. If it does, the IAC valve may be functioning properly. If not, use the next method to check and see if it needs to be cleaned.

Part 3 of 4: Visually inspecting the valve

Step 1: Gain access to the IAC valve. Consult the vehicle service manual for the location of the IAC valve on your vehicle.

Step 2: Disconnect IAC valve. Locate the IAC valve electrical connector and unplug the IAC valve.

Step 3: Remove the IAC valve from the vehicle. Use the procedure detailed in the vehicle service manual to remove the IAC valve.

Step 4: Inspect the IAC valve. Inspect the valve and the mounting location for carbon build-up, rust or dirt. Inspect the IAC valve pintle and mounting location for damage.

Step 5: Clean the IAC valve and bypass channel. Use a carbon cleaner or intake cleaner solvent to remove buildup and dirt from the IAC valve. Use the straw that comes with the spray can to clean the IAC valve mounting location and bypass passage as well.

  • Warning: Do not use metal wire brushes to clean the valve or bypass circuit. Scratching the walls or the pintle with a metal wire brush can change the IAC valve function. Step 6: Install the IAC valve. Install the IAC valve with a NEW seal. Using the old seal may cause a vacuum leak or a coolant leak on vehicles where coolant runs through the IAC valve. Step 7: Start the engine. If you used a lot of solvent, the engine may run rough momentarily as it takes in and burns off any leftover solvent. After a brief period of running rough, the idle should return to normal.

Part 4 of 4: Using a multimeter to check IAC valve resistance specification

Step 1: Gain access to the IAC valve. Consult the vehicle service manual for the location of the IAC valve on your vehicle.

Step 2: Disconnect IAC valve. Locate the IAC valve electrical connector and unplug the IAC valve.

Step 3: Remove the IAC valve from the vehicle. Use the procedure detailed in the vehicle service manual to remove the IAC valve.

Step 4: Inspect the IAC valve. Inspect the valve and the mounting location for carbon build-up, rust or dirt. Inspect the IAC valve pintle and mounting location for damage. Rules these items out before condemning the IAC valve.

Step 5: Check resistance of IAC valve. Use the specification listed in the vehicle service manual for the IAC valve and follow the guideline on how to test the valve using a digital multimeter against the electrical terminal pins on the IAC valve electrical connector. If the reading falls within specification, the valve should be electronically sound and the fault lies somewhere else. If the reading is not within the specification, replace the unit with a new one.

  • Note: The new IAC valve may or may not come with a new seal. Remember to replace the seal any time a sealed part is removed from the engine to avoid a vacuum leak or a coolant leak where coolant runs through the IAC valve body.

A professional technician equipped with a professional grade vehicle scan tool may be able to plug into the vehicle and command the IAC valve through the scanner to check operation. If you have followed all methods of testing and are still stumped, consider hiring a professional technician to take a look. The technicians we have to offer here at YourMechanic are happy to make house calls.


Next Step

Schedule Check Engine Light is on Inspection

The most popular service booked by readers of this article is Check Engine Light is on Inspection. Once the problem has been diagnosed, you will be provided with an upfront quote for the recommended fix and receive $20.00 off as a credit towards the repair. YourMechanic’s technicians bring the dealership to you by performing this job at your home or office 7-days a week between 7AM-9PM. We currently cover over 2,000 cities and have 100k+ 5-star reviews... LEARN MORE

SEE PRICING & SCHEDULING

The statements expressed above are only for informational purposes and should be independently verified. Please see our terms of service for more details

Recent Check Engine Light is on Inspection reviews

Excellent Rating

(7,400)

Rating Summary
6,879
249
58
41
173
6,879
249
58
41
173

Keven

7 years of experience
118 reviews
Keven
7 years of experience
Mazda Protege L4-1.5L - Check Engine Light is on - Tacoma, Washington
Kevin is a focused man who has no doubt about his job. Quiet,friendly,respectful. I have nothing but the best to say. Thanks again for your help.
Toyota Sienna - Check Engine Light is on - Spanaway, Washington
He was very efficient, very knowledgeable and very sweet. Thank you for your time and your patience with my vehicle.

David

13 years of experience
393 reviews
David
13 years of experience
Chevrolet Malibu L4-2.4L Hybrid - Check Engine Light is on Inspection - Marietta, Georgia
He explained everything to me in detail on what I needed to know about my car. Definitely recommend him for getting the job done.

Raymond

37 years of experience
402 reviews
Raymond
37 years of experience
Toyota Camry L4-2.2L - Check Engine Light is on - Las Vegas, Nevada
Raymond did a great job however I was charged for two hours labor and he was only here less than 45 minutes. should I expect this on future appointments with your mechanic ?

Shane

17 years of experience
217 reviews
Shane
17 years of experience
Volkswagen EuroVan V6-2.8L - Check Engine Light is on Inspection - Austin, Texas
Arrived on time. Explained the issues he found and how he will fix it. I have parts on order and will schedule an appointment to have Shane install the parts when they arrive.

Need Help With Your Car?

Our certified mobile mechanics make house calls in over 2,000 U.S. cities. Fast, free online quotes for your car repair.

GET A QUOTE

Related articles

How Long Does a High Pressure Safety Valve Last?
Among Among the most important parts of an engine is the oil that it contains. Without the right amount of oil pressure in an engine, it will be hard for all of the moving parts to get the lubrication that...
How Long Does a Proportioning / Combination Valve Last?
Modern Modern brake systems are far more advanced than they were even a few years ago. Today, most cars come with front disc and rear drum brakes (although many also come with four-wheel disc brakes). In a disc/drum combination system,...
How to Replace an Air Conditioning Expansion Valve (Orifice Tube)
Car air conditioning systems need the expansion valve, or orifice tube, replaced if the AC is blowing warm air or the AC system is broken.

Related questions

Timing off with engine.
There are multiple issues that can cause an engine to run rough and timing is one of them. However, confirming that the timing is off may take some extensive testing or removal of several parts of the engine to access...
Getting codes P0161 and P0325
Hello. Both codes indicate a circuit failure, and I suspect that the connectors were not properly reconnected after changing the valve cover gasket. These codes are not likely actual part failures. It is a very tight squeeze to get in...
Ignition coil bolt can't bolt down
The valve cover on that side of the engine has the broken bolt boss. The only way to secure the coil is to either replace the valve cover so you have the bolt boss to bolt down the coil, or...

How can we help?

Our service team is available 7 days a week, Monday - Friday from 6 AM to 5 PM PST, Saturday - Sunday 7 AM - 4 PM PST.

1 (855) 347-2779 · hi@yourmechanic.com