P0112 - IAT sensor low input code - can't figure out | Ford Explorer Forums - Serious Explorations

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P0112 - IAT sensor low input code - can't figure out

dpappyp

Member
Joined
February 11, 2008
Messages
17
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City, State
milwaukee
Year, Model & Trim Level
2000 Mountaineer
If I clear the code it might stay off for a minute or might stay off for days but it always comes back. Sometimes the code even clears itself, but soon enough it pops up again. I tried cleaning the MAF (iat is apart of it) and then replaced the maf altogether, but no luck. I was reading the live IAT sensor temperature reading and yes it does erroneously bounce up to its maximum reading, somewhere north of 300 degrees fahrenheit (corresponds to the low voltage input error). I did notice that in general it reads a normal temp at idle, but almost always maxes out when accelerating and then drops back down when I get up to speed. I tried researching that specific symptom but came up empty. Has anyone else seen this issue? Car runs fine, but I really need to get this figured out before an upcoming emissions test. Could it be as simple as a wire shorting out somewhere in the harness? The relation to acceleration makes me think otherwise, however. Any clues would greatly appreciated.

FIXED:
Well I went with the idea that there was a short somewhere in the harness, but trying to locate it in the bundled wiring harness would have been an absolute mess. So, instead I located the corresponding pin at the PCM for the IAT (pin 39) and chopped off the gray wire going to it. Then I chopped the wire at the IAT sensor and ran a new wire from the PCM pin 39 to the IAT sensor. No more temperature reading fluctuating with acceleration, it's steady regardless of air flow or jostling. Looks like I'll be passing my emissions test.
 



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the iat is not a part of the maf if i am not mistaken,, , it is in the inlet throat , hooked up with a 2 wire connection, and it looks like this,
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check the whole inlet track, and see if you can find that, they are cheap , i think under $20,,
 






Correct, the intake air temp sensor is not part of the MAF sensor. Check the connection, make sure it's clean and tight. If it still throws the code, change the sensor.
 






integrated MAF/IAT sensors

The early model Explorers had separate MAF sensor and IAT sensor but my 2000 has an integrated MAF/IAT sensor assembly. The integrated assembly has six wires in the connector. The outer wires (gray and gray/red) are for the IAT sensor. Possibly the wiring harness is stretched tight and when the engine accelerates the engine movement tightens the harness breaking the connection. Or maybe one of the wires is broken internally. Also check the connection at the PCM. The connection could be loose or the pin for the wire could be pushed back not making good contact.
 






short not open

. . . I was reading the live IAT sensor temperature reading and yes it does erroneously bounce up to its maximum reading, somewhere north of 300 degrees fahrenheit (corresponds to the low voltage input error). . . Could it be as simple as a wire shorting out somewhere in the harness? . . .

After thinking about the problem more I agree that there is probably a short in the wire instead of an open as I previously posted. The IAT sensor is a thermistor that decreases resistance as temperature increases. The PCM provides a reference voltage (probably 5 volts for an open circuit) on the gray wire to the IAT sensor. As the thermistor resistance decreases (temperature increases) the reference voltage will decrease. The gray/red wire is a common return for most of the sensors (TPS, CMP, DPFE, ECT, etc). The PCM uses an isolated return for the sensors instead of ground. The IAT temperature vs resistance table in my Haynes manual lists a maximum temperature of 248 degrees F for a resistance of 1.18 Kohms and a minimum temperature of 50 degrees F for a resistance of 58.75 Kohms. The 2000 MY OBD II System Operation Summary states that "Typical analog sensor check malfunction thresholds: Voltage < 0.20 volts or voltage > 4.80 volts". A short to ground for the gray/red wire would have very little (if any) affect on the reading. A short to ground or signal return for the gray wire would drop the voltage to zero corresponding to a temperature exceeding 248 degrees F (you read 300 on the scanner). The short could be anywhere from the MAF sensor connector to the PCM pin 39. It could also be internal to the MAF/IAT sensor but you said you replaced it. Closely examine the MAF/IAT sensor connector to make sure the gray wire pin is not shorting to the adjacent pin.
 






Thanks for the 2 cents. Yea a short seems the most likely cause and thats what I want it to be the least. The connector and harness both appear to be fine, but I suppose I could try putting in a jumper wire, bypassing the harness.
 






FIXED: see first post edit
 






For what it's worth: I was battling a similar intermittent low IAT voltage symptom for months. When that happened, my scanner was showing the intake air temperature shooting up to 200 deg or so. Finally discovered that a bracket which supports the harness at the back of the engine had a very sharp edge that penetrated the insulation and was touching the IAT wire.
 






Hmm, better check mine then, thanks for the tip. My concern is something damaged that IAT wire and it's in a bundle with a bunch of other wires, so it might be only a matter of time before another one shorts out.
 












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