How To Test The Intake Air Temp (IAT) Sensor (Toyota 1.8L)

In this tutorial I'm gonna' show you how to test the intake air temperature (IAT) sensor of your Toyota equipped with a 1.8L 4 cylinder engine.

The intake air temperature (IAT) sensor is part of the mass airflow (MAF) sensor assembly and it can be easily tested with either a scan tool or a multimeter and I'll show you how.

You can find this tutorial in Spanish here: Cómo Probar El Sensor IAT (1.8L Toyota Corolla) (at: autotecnico-online.com).

Symptoms Of A Bad IAT Sensor

When the intake air temperature (IAT) sensor on your Toyota goes bad, the check engine light (CEL) will immediately illuminate to let you know that there's a problem and that a diagnostic trouble code has set to indicate a problem.

Here are some of the specific symptoms you'll see/have:

1. Diagnostic trouble codes:
   • P0110: Intake Air Temperature Circuit Malfunction.
   • P0112: Intake Air Temperature (IAT) Sensor Circuit Low Voltage.
     • When your scan tool reports this DTC, the IAT sensor is reporting a temperature of 284 °F (140 °C) or hotter.
   • P0113: Intake Air Temperature (IAT) Sensor Circuit High Voltage.
     • When your scan tool reports this DTC, the IAT sensor is reporting a temperature of -40 °F (-40 °C) or colder.
2. Bad gas mileage.
3. Black smoke coming out of the tailpipe.
4. Won't pass the emissions test.

What Tools Do I Need To Test The IAT Sensor?

Here's a basic list of tools you'll need:

1. A multimeter.
   • If you need to upgrade or buy a multimeter, check out my recommendation: Buying A Digital Multimeter For Automotive Diagnostic Testing (found at: easyautodiagnostics.com).
2. Scan tool.
   • Don't have one? Check out my recommendation: Abe's Scan Tool Recommendation.
3. Wire piercing probe.
   • Although this tool is not an absolute must, if you do buy one, you'll realize just how easy it makes testing the voltages inside the wires.
   • If you need to see what this tool looks like, you can see it here: Wire Piercing Probe.

What Does The IAT Sensor Do?

The intake air temperature sensor's job is to take a measurement of the temperature of the incoming air (that the engine is breathing) and then send this info to the PCM.

Once the PCM gets it, it uses this info to better calculate the mass of the air entering the engine.

To get into a little more detail: your Toyota's PCM needs to know two very important things (among several) to calculate the correct amount of fuel to inject for the amount of air the engine is breathing.

These two things are:

1. Temperature of the incoming air.
2. The flow rate of the air the engine is breathing.

It all boils down to the fact that temperature has a direct effect on air density (mass) and since the MAF sensor can only measure the flow rate of air passing thru' it, the IAT sensor helps the PCM to further ascertain the precise amount of air entering the engine (and this is one of the reasons why the intake air temperature sensor is part of the mass airflow sensor assembly).

This in turn helps the PCM maximize everything from fuel consumption to performance.

OK, these are the circuit descriptions of the mass air flow (MAF) sensor and the intake air temperature (IAT) sensor:

MAF Sensor Connector Pin Out
Pin	Wire Color	Description
1	Black	Fused power (12 Volts)
2	Blue w/ White stripe	MAF sensor Ground (PCM)
3	Green	MAF sensor signal
4	Yellow w/ Black stripe	Intake Air Temp (IAT) Sensor
5	Brown	Intake Air Temp (IAT) Sensor

As you're probably already aware, the MAF sensor on your Toyota has 5 wires coming out of its connector. We only need to be concerned with 2 of the 5 wires since the other 3 belong to the MAF sensor that is part of the assembly.