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Originally Posted by flir67
I'm having the exact same problem on my 99 altima se. my light just came on a few weeks ago. I replace the air senor in intake tube before the air filter but still no go.
I've read that it might be a short in the wires. how do you test to see if the air sensor is working hot and cold?
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My Haynes manual covers this in slight detail as follows...
1. Disconnect the two-wire electrical harness connector from the Intake Air Temperature (IAT) Sensor (near and outboard of the battery, on top of the air intake duct, upwind of the air filter assembly).
2. Turn the ignition key switch to the "ON" position without starting engine.
3. With a volt meter, carefully probe/read the DC voltage across the two socket contacts in the female connector (wire harness connector). This is the reference voltage for the sensor supplied by the ECM, it should be approximately 5 vdc. The picture in the manual shows 4.88 vdc, my '01 Altima read at 4.89 vdc on my meter. If this reading is substanially lower but not zero, it may be a few things: ECM internal problem, short circuit between conductors or short to ground, or one or more poor connection(s) between ECM and IAT connector. Haynes sugguests it is usually wiring/connector-related, and it should be taken to a Nissan shop. If the voltage is dead zero, then the problem is likely to be an open circuit in the wiring between the ECM and the IAT connector, or a toasted ECM--check other ECM functions or ECM fuse to confirm this.
4. Turn the key switch to "OFF."
5. If the IAT reference volts are good , then check the IAT sensor as follows:
Probe the IAT sensor (male) connector pins carefully with an ohm meter, try not to bend the pin contacts. Read the resistance across the two pins on the sensor. It should be in the range of 1K-3K Ohms. If the car is cold, the reading should be close to or in the higher half of this range. It's hard to say exactly what it should be, because Haynes doesn't provide a table or chart for the exact resistance v. temperature relationship. After checking a cold car, then start the engine and let the car warm up to normal operating temperature. Repeat the resistance measurement on the IAT sensor as before. Now the resistance should be substantially lower. If the ohm meter indicates either an open circuit (or very high resistance: >.5M Ohms), or a short circuit (very low resistance: < 200 Ohms) across the IAT sensor, then the sensor itself is likely defunct.
(For this check I didn't bother following Mr. Haynes' method to run the engine to warm the car. I removed the IAT sensor from the intake duct and read its resistance while partially submerging it alternately in a cups of cool and warm water. Doing this yielded results quickly, and let me clean the sensor; mine read about 2.2K Ohms cool and about 1.5K Ohms warm. If I had to do it again, I would simply blow warm and cool air on it using a hair dryer.)
I hope this helps, good luck, and let me know if you have any success resetting your "Service Engine Soon" (MIL) light.