2000 Chevrolet Malibu Question about IAT sensor

The confusion is in how the service manual describes how the circuit works. The sensor is fed with 5.0 volts, but you are only going to find that at the sensor if you unplug it. Doing that is going to set a diagnostic fault code. The opposite to that "open circuit" is if you grounded that wire, then you'd find 0 volts. That will also set a fault code. When the sensor is good and plugged in, you'll find between 0.5 and 4.5 volts. The higher the temperature, the lower the voltage. Anything outside of that voltage range is what triggers a fault code.

Temperature sensors have an extremely low failure rate because there is just one component in them. Almost all fault codes are a result of corrosion on the connector terminals or a corroded splice. In this case the signal wire is specific to each sensor but many sensors share a common ground wire. It isn't exactly a ground wire though. It goes to ground through the Engine Computer so the computer can monitor it for a good connection. The splice in that wire is what can corrode. Consider that a good suspect when you suddenly get multiple codes for multiple sensors. Other, non-temperature sensors share a common 5.0 volt feed wire. You'll get multiple fault codes for them when there's a break in that wire.

With it unplugged and the ignition switch on, you must find 5.0 volts on the feed wire, and you'll have 0.2 volts on the ground wire.

If you don't have 5.0 volts, there has to be a break in that wire. There's no splice in that one because that wire is specific to that sensor. When you find a little voltage there, it's getting there through some corrosion. A digital voltmeter needs almost no current to read the voltage, and that's what corrosion does. It prevents current from flowing. If you'd use a test light, which would light up dimly on 5.0 volts, you'll see it doesn't light up at all on your wire. Test lights need current to operate, and current can't get through the corrosion.

That is different than a total break in the wire. The voltmeter would not see any voltage then. You didn't say what you are getting for voltage. Is it 0 volts or something higher?

Dandy. You have a break in that wire between the sensor and the Engine Computer. Unplug that sensor and measure the voltage again. If you find close to 5.0 volts now, it's not a solid break as in a cut wire. You're looking for the type of break that can allow a little current to flow yet, and for this type of circuit that usually means corrosion. By far the most common cause of that is where someone poked a hole in the wire's insulation to take a reading weeks or months ago, and moisture came in and caused that corrosion. A second rather common place to look is right by the connector where the wires come out. If you tug lightly on each wire, a corroded one will act like a rubber band instead of a wire. Often the clue is the insulation will be a lighter color around the corroded spot.

The 2.7 volts you mentioned you had earlier is a proper voltage, and since it is intermittently there and not there, that also suggests a corroded spot and not cut in the wire.

With the connector unplugged, you gotta have 5.0 volts on the feed / signal wire. Follow that one back to the computer and look for a break or corroded connector pin. I'm sorry I can't tell you which terminal that is at the computer, but those terminals are typically very reliable. Also, that 5.0 volts feeds a lot of other sensors through different terminals, so if there was a problem with that internal power supply inside the computer, you'd be getting fault codes for a lot of other sensors. I think you're going to find a break in the wire.

I'm confused.

"but dorve it and light came back on again after I had removed it"

You unplugged it under the hood while you were driving?

Dandy. 3.0 volts is fine, but you're saying the Check Engine light turned on again? Did you check the codes to see if it's setting the same one? If it is, the easiest is to drive with a scanner connected so you can watch that voltage to see what it does. You can do that with a voltmeter too. You can hook it under the wiper arm or run the leads inside the car with a bunch of clip leads Actually, you need both for the whole diagnosis. If you see that voltage go to 0 volts at the sensor, and the scanner shows 5.0 volts, there's a break in that wire. If both of them go to 0 volts, that wire is grounded, and for a grounded wire to be intermittent, it typically has to be rubbing on the edge of a sharp bracket or is laying on hot engine or exhaust parts.

Dandy. Happy to hear you solved it. Come back to see us again.