Circuit Description

The Intake Air Temperature (IAT) sensor, mounted on the Mass Air Flow (MAF) meter, monitors the IAT. The IAT sensor has a built in thermistor with a resistance that varies according to the temperature of the intake air. When the IAT is low, the resistance of the thermistor increases. When the temperature is high, the resistance drops. These variations in resistance are transmitted to the ECM as voltage changes (see Figure) The IAT sensor is powered by a 5V supply from the THA terminal of the ECM, via resistor R.

Resistor R and the IAT sensor are connected in series. When the resistance value of the IAT sensor changes, according to changes in the IAT, the voltage at terminal THA also varies. Based on this signal, the ECM increases the fuel injection volume when the engine is cold to improve driveability.

HINT:

When any of DTCs P0110, P0112 and P0113 are set, the ECM enters fail-safe mode. During fail-safe mode, the IAT is estimated to be 20°C (68°F) by the ECM. Fail-safe mode continues until a pass condition is detected.

Fig 1: Intake Air Temperature-Resistance Graph

Courtesy of TOYOTA MOTOR SALES, U.S.A., INC.

Fig 2: DTC Trouble Detection Chart (DTC P0110/0112/0113)

Courtesy of TOYOTA MOTOR SALES, U.S.A., INC.

HINT:

When any of these DTCs are set, check the IAT by selecting the following menu items on a hand-held tester: DIAGNOSIS/ENHANCED OBD II/DATA LIST/ALL/INTAKE AIR. If using an OBD II scan tool, refer to the instruction manual.