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.
TEST DESCRIPTION:Number(s) refer to circled number(s) on the diagnostic chart.
Code 23 will set if:
* a signal voltage indicates an intake air temperature below -40 degrees C for 4 seconds. Due to the conditions necessary to set code 23, 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, due to the ECM transmitting "default" (substitute) values when the fault is present. A code 23 will set. due to an open sensor, wire, or connection. this test determines if the wiring and ECM are OK.
2. If the resistance is greater than 25,000 ohms, inspect the air cleaner assembly for the presence of ice. If OK, replace the sensor.
SOURCE: GM SERVICE MANUAL
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