Intake air temperature sensor (IAT)

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By Reinier (Contact Me)
Last Updated 2021-09-06
Automobile Repair Shop Owner

What Does the Intake air temperature sensor (IAT) Do?

 

The sole purpose of the Intake Air Temperature (IAT) sensor is to measure the temperature of the air that enters the engine via the air filter.

Why is the Intake air temperature sensor (IAT) Needed?

For a modern engine to operate efficiently, it needs input data from many engine sensors that measure and/or monitor various environmental and operational conditions, many of which can vary a great deal during a single trip. Moreover, since modern engines are required to emit maximum allowable amounts of harmful exhaust emissions under all operating and environmental conditions, some sensors play critically important roles in the management of fuel delivery strategies to save fuel and limit exhaust emissions.

One such critically important sensor is the Intake Air Temperature sensor, which supplies the PCM (Powertrain Control Module) with input data about the temperature of the intake air. In practice, cold air is always denser than hot air, meaning that more cold air fits into the inlet manifold than hot air, which affects how the PCM calculates the volume of fuel each injector injects into the cylinders.

Thus, if the intake air is hot, there is less air available to mix with the fuel, meaning that the air/fuel mixture will be rich*. Conversely, if the intake air is cold, there will be more air available to mix with the fuel, meaning the air/fuel mixture will be lean.**

The ideal air/fuel mixture on gasoline engines is known as a “stoichiometric mixture”, and consists of one part of fuel to 14.7 parts of air, which allows all the fuel to be combusted using all of the available air.

*In a rich mixture, there will be more than one part of fuel to 14.7 parts of air.

** In a lean mixture, there will be less than one part of fuel to 14.7 parts of air.

So, since the density of air is a function of its temperature, the PCM uses input data from the IAT sensor to make appropriate adjustments to the volume of fuel the injectors deliver to the cylinders to maintain the air/fuel mixture at, or as close to the ideal 14.7: 1 ratio as possible. The practical effect of this strategy is reduced fuel consumption and exhaust emissions while maintaining maximum engine performance at all times under all operating and environmental conditions.

How Does the Intake air temperature sensor (IAT) Work?

All IAT sensors are essentially thermistors, which are sensors whose electrical conductivity changes in response to changes in temperature.

So, in practice, the PCM supplies the IAT sensor with a 5-volt reference voltage, which current flows through the sensing element. As the temperature of the intake air changes, i.e., gets hotter or colder, the conductivity of the sensing element changes in direct proportion to changes in temperature, which, in turn, causes changes in the current that flows back to the PCM.

The PCM interprets the changes in the current as temperature changes, and depending on the direction of change at any given moment, the PCM will either increase or decrease the amount of fuel the injectors deliver to the cylinders.

Note, though, that the temperature of the intake air is directly proportional to its density. Therefore, the PCM will compare input data from the IAT and Manifold Absolute Pressure (MAP) sensor to verify the plausibility of inputs from both sensors.

Where is the Intake air temperature sensor (IAT) Located on the Engine?

This image shows the location (circled) of the IAT sensor in the inlet duct of a VW application. Note that while the IAT sensor on many applications is located somewhere in the inlet tract, the actual locations of IAT sensors vary greatly between vehicle makes and models. Also, note that on vehicles that are fitted with MAF (Mass Airflow) sensors, the IAT is typically incorporated into the MAF sensor, meaning that the entire MAF sensor must be replaced when the IAT sensor fails.

Thus, to avoid misdiagnoses, and the possible replacement of the wrong sensor, we strongly recommend that you consult reliable service information for the affected application to locate and identify the IAT sensor correctly. This is particularly important on V-type engines, some of which have a dedicated IAT sensor for each bank of cylinders.

What Does the Intake air temperature sensor (IAT) Look Like?

This image shows an example of an Intake Air Temperature sensor. Note, though, that where IAT sensors are concerned, no general pattern applies to their design. For instance, a retaining screw holds down some IAT sensors (like this example) some sensors simply “plug” into the inlet tract, while still others screw into metal parts of the inlet tract.

Therefore, we recommend that you remove the old sensor and use it as a sample when you purchase a replacement to avoid purchasing the wrong or an unsuitable IAT sensor.

What are the Symptoms that the Intake air temperature sensor (IAT) is Bad?

The most common symptoms of failed, failing, or defective IAT sensors are largely similar across all applications, and could include one or more of the following-

  • Stored trouble code(s) and illumined warning light, which may or may not flash
  • Poor fuel economy and reduced engine performance-especially upon acceleration
  • A hard starting condition may be present
  • The engine may run roughly or may exhibit misfires under all or some operating conditions
  • The idling speed may fluctuate, or the engine may idle roughly
  • In some cases, and depending on both the application and the nature of the problem, the PCM may initiate and maintain a limp mode or default fuelling strategy that will persist until the fault is found and corrected

Note that all or most of the above symptoms could also be caused by defects in parts, sensors, systems, and components that are unrelated to the IAT. Therefore, we do not recommend that you replace the IAT sensor unless or until you have investigated and/or resolved all other non-IAT sensor-related trouble codes that may be present on the affected vehicle.

Note that while defective IAT sensors could produce abnormal fuel trim values, abnormal fuel trims are a symptom of one or more problems- as opposed to being the cause of issues like excessive fuel consumption, increased emissions, and/or reduced engine performance.

How do you test the Intake air temperature sensor (IAT)?

The problem with IAT sensor failures is that not all failures or malfunctions will always set one or more fault codes. Generally, only wiring issues like short or open circuits will set fault codes, which means that scan tools, even advanced scan tools, will not always detect failing and/or malfunctioning IAT sensors.

In such cases, secondary tests, such as fuel trim analysis and volumetric efficiency tests are required to exclude some causes of some symptoms. However, the problem with these tests is that few non-professional mechanics possess the skill, knowledge, and equipment to interpret these kinds of test results accurately or correctly.

Moreover, since some direct tests to determine the efficiency of IAT sensors involve performing electrical resistance tests on both the suspect sensor and its associated wiring, we do not recommend this procedure for non-professional mechanics. Getting these kinds of tests wrong can fatally damage several control modules, including the PCM. In addition, resistance values are directly proportional to the exact temperature of the sensor, meaning that reliable, OEM-level service information is required to interpret test results.

Given the above, we recommend that you seek professional assistance with diagnosing and the (possible) replacement of suspect IAT sensors to prevent damage to control modules and other electronic systems.

How do you replace the Intake air temperature sensor (IAT)?

In most cases, replacing an IAT sensor is a simple procedure that should fall well within the capabilities of most non-professional mechanics.

Once you have identified and located the IAT sensor, disconnect the electrical connector, and use suitable tools to remove any retaining screws or spring clips. Take care not to lose these, so set them aside well out of the way.

Next, remove the old sensor from the inlet ducting, and insert the replacement. Insert all retaining screws or spring clips, but make sure not to over tighten screws as this could damage the new sensor. Reconnect the electrical connector, and make sure the two parts lock together securely to prevent continuity issues.

As the final step, secure all wiring to prevent wiring from rubbing and/or chafing against engine components.

NOTE: in some cases, it is possible to restore the IAT sensor’s operation by cleaning the sensing element with an approved solvent. Be aware, however, that brake cleaners, and especially brake cleaners in aerosol form, are NOT suitable for this purpose since these products will remove a special anti-static coating on the sensing element, thus rendering the sensor useless. The best products to use for cleaning sensors are products that are clearly labeled as “sensor safe” or products that are formulated specifically to clean MAF (Mass Airflow) sensors.