The intake Air Temperature (IAT) sensor is a thermistor. The ECM applies a voltage (about 5 volts) on circuit (tan wire) to the sensor. When air is cold, the sensor (thermistor) resistance is high, therefore, the ECM will measure a high signal voltage. If the air is warm, the sensor resistance is low, and the ECM will measure a low voltage.
TEST DESCRIPTION:Number(s) refer to circled number(s) on the diagnostic chart.
Code 25 will set if:
* Signal voltage indicates an intake air temperature greater than 135 degrees C (275 degrees F)
* Vehicle speed is greater than 35mph
* Both of the above requirements are met for at least 16 seconds.
Due to the conditions necessary to set a code 25, the "service engine soon" light will only stay "ON" while the fault is present.
1. A "scan" tool may not be used to diagnose this fault to the ECM transmitting "default" (substitue) values when the fault is present. If voltage is above 4 volts, The ECM and wiring are OK.
2. If the resistance is less than 185 ohms, replace the sensor.
(IAT): The intake air temperature sensor is a thermistor, a resistor, which changes value based on the temperature of air entering the engine. Low temperature produces a high resistance (100,000 ohms at -40 degree C/-40 degree F), while high temperature causes low resistance (70 ohms at 130 degree C/ 266 degree F). The ECM supplies a 5 volt signal to the sensor through a resistor in the ECM and measure the voltage. the voltage will be high when the incoming air is cold, and low when the air is hot. by measuring the voltage, the ECM calculates the incoming ait temperature. the IAT sensor is also used to control spark timing.
the scan tool display temperature of the air entering the engine and should read close to ambient air temperature when engine is close to ambient air temperature when engine is cold, and rise as under hood temperature increases. if the engine has not been run for several hours (overnight). the IAT sensor circuit should set a code 23 or 25. the code charts also contain a chart to check for sensor resistance values relative to temperature.
SOURCE: GM SERVICE MANUAL.
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