TEST 3: IAT Sensor Circuit Low Voltage

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

You've reached this step because in TEST 1 your scan tool confirmed that the intake air temperature (IAT) sensor's PID is reporting an extremely hot temperature that doesn't correspond to the actual ambient air temperature around the vehicle.

This extreme hot temperature also confirms trouble code P0112 that's lighting up the check engine light on your Toyota's instrument cluster.

In this test step, we're gonna' check to see if the IAT sensor wires have shorted together.

This is what you'll need to do:

  1. Connect your scan tool and get to its Live Data mode.
  2. Scroll down to the PID for the IAT sensor.
    1. 284 °F (140°C) or higher temperature reading should still be present.
  3. Now, remove the black plastic loom from the MAF sensor wires (about 6 inches from the connector).
  4. Remove any black electrical tape that may be wrapped around the wires.
    1. What you're looking for, as you remove this black electrical tape is to see if the wires are shorted together.
  5. If the IAT sensor wires are shorted together you'll notice that:
    1. The insulation has peeled of exposing the copper inside the wires.
    2. At certain points, the copper wires are (or were) touching each other.
    3. NOTE: the IAT sensor wires are the one labeled with the numbers 4 and 5 in the image viewer.

Let's interpret your test results:

CASE 1: The IAT sensor wires were shorted together- This tells you that you that the connector is bad and needs to be replaced.

You can verify that this is the repair solution by separating the wires that are shorted together and then checking the intake air temperature PID (on your scan tool) to see if the temperature has gone from 284 °F (140 °C) to a temperature that is ±10 °F of ambient temperature.

CASE 2: The IAT sensor wires WERE NOT shorted together- This tells you that the MAF sensor connector is OK.

The next step is to unplug the IAT sensor from its connector and see if the temperature reading on your scan tool changes to -40 °F (-40 °C). For this test, go to: TEST 4.

TEST 4: Unplugging The IAT Sensor

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

So far you've confirmed that:

One: The mass airflow sensor's connector doesn't have any wires that are shorted together (TEST 3)

Two: That the IAT sensor is showing an extreme hot temperature of 284 °F (140 °C) (TEST 2).

The next step is to make sure that the PCM isn't bad and we can very easily do this by disconnecting the MAF sensor's connector and seeing if the PCM reacts.

If the PCM and the wires between the IAT sensor and the PCM's connector are okay, then your scan tool will now display an extreme cold temperature of -40 °F (-40 °C) for the IAT sensor's PID.

OK, here's what you'll need to do:

  1. Connect your scan tool to your vehicle and:
    1. Turn the Key On.
    2. Go to Live Data mode.
  2. Scroll down to the IAT sensor PID.
    1. The IAT sensor should be reading 284 °F (140°C) or higher.
  3. Disconnect the mass air flow (MAF) sensor from its connector.
  4. Once you disconnect the MAF connector the IAT sensor PID should read:
    1. -40 °F (-40°C) or colder.
    2. If you check for codes, you should see a P0113: Intake Air Temperature (IAT) Sensor Circuit High Voltage.
  5. When done, turn the key Off and interpret your results below:

CASE 1: Your scan tool showed -40 °F (-40 °C) or colder - This is the normal result from unplugging the IAT sensor (MAF connector) and tells you that the PCM and the wires between the MAF sensor connector and the PCM's connector are OK.

The next step is to check the resistance of the IAT sensor itself (unplugged from its connector). For this test, go to: TEST 6.

CASE 2: Your scan tool DID NOT show -40 °F (-40 °C)- This tells you that the there's something wrong with the wiring harness or the PCM itself.

Now, checking the wiring between the PCM or the PCM itself is beyond the scope of this tutorial but you have at least eliminated the intake air temperature (IAT) sensor inside the MAF sensor as bad.

Before checking anything else, I suggest that you do TEST 8.

Toyota Vehicles:

  • Celica 1.8L
    • 2000, 2001, 2002, 2003, 2004, 2005
  • Corolla 1.8L
    • 2003, 2004, 2005, 2006, 2007, 2008
  • Matrix 1.8L
    • 2003, 2004, 2005, 2006, 2007, 2008

Toyota Vehicles:

  • MR2 Spyder 1.8L
    • 2000, 2001, 2002, 2003, 2004, 2005