98 Pathfinder HAs Two IAT Sensors, Will it Still Work With Just One?

[Alibireason]

I went sensor replacement crazy recently (Bad MPG) and replaced the IAT sensor located under the hood latch. I didn't check actual recorded temp before or after. Soon after was at junkyard looking at Pathys and saw others had TWO sensors right next to each other under hood latch. Looked back at mine and behold the previous owner had disconnected and taped up IAT sensor harness. I got one from the junkyard and put it on.
Cut to months later I'm checking temp on OBD2 scanner and on a 50*F day it was reading 120*F. I assume an inaccurate reading this high is not good for MPG, correct?

Today I tested both harnesses for voltage and continuity. Harness that had been previously left empty had no voltage and no continuity. Harness with new sensor tested at just under 5 volts and the beautiful bell of continuity rang.

I assume previous owner knew this thus only the one being connected.

My question is: Do I need both of those sensors working correctly to ensure a proper reading? Will the one good one do the trick. If no what would be my next step in figuring out why the other sensor harness isn't working?

Thank you!

[VStar650CL]

Neither one of those is IAT, the IAT sensor is integrated with the MAF. One is ambient temperature for the HVAC system, the other is ambient temperature for the display in the rearview.

[VStar650CL]

Well, strike that, on a '98 the IAT wasn't integrated yet, but it's still on the intake duct at the LHF of the engine, not behind the grille. The two sensors behind the grille are what I said.

[Alibireason]

The Haynes manual shows that the IAT sensor is next to left side headlight for 3.3. On mine there is nothing. When I bought the car it had RHS bumper damage and a broken coolant overflow tank, which have both been replaced. Maybe that same accident broke the original setup and it was modified? Below the hood latch are the two identical sensors, and below them is I believe the ambient temp sensor.

[Alibireason]

Hear me out. My 1998 Pathfinder SE doesn't have rearview temp display, and I took the mirror off and there is not a harness connector up there to connect one. Since one of the two connectors is for that rearview temp, would that explain why one of them has zero voltage/continuity, because it goes nowhere? And the one that works in the HVAC? And that the sensor near bottom of radiator is the ambient air temp sensor?
Just thinking out loud.

Also, I tried to remove harness from the bottommost sensor (upper two were easy) and it would not come off. I know these old parts are brittle. Any advice?

[VStar650CL]

The Haynes manual shows that the IAT sensor is next to left side headlight for 3.3. On mine there is nothing.

That's what I recall too, so maybe someone removed it. The IAT has to be in the intake air stream or it it's measuring ambient and not intake. If you put a scanner on it and find the IAT reading -22 or -40 degF then it's missing or open. That will also explain your horrible mpg, because the car thinks it's in Alaska.

Since one of the two connectors is for that rearview temp, would that explain why one of them has zero voltage/continuity, because it goes nowhere?

Yes, both of those sensors are "balance" setups with a pullup resistor inside the controller, the sensor acts as a pulldown to produce a variable voltage from the ratio of pullup:pulldown. So if there's no voltage then there's no controller attached.

Also, I tried to remove harness from the bottommost sensor (upper two were easy) and it would not come off. I know these old parts are brittle. Any advice?

Hit it with WD40 or PB Blaster and come back to it in an hour. The latches get filled with crud over years of use and become very stubborn. Some lubrication will generally cut way down on the force needed and will reduce the chances of breaking an embrittled latch.

[Alibireason]

If my car is missing the IAT sensor, would you know where the OBD2 scanner is getting an IAT temp of 120*F from. I recall from past live scans of it being at 108*F, so I assume a fluctuation means something somewhere is giving the ECU that info. BUT
FROM WHERE? This is really frustrating because I feel I am at the LAST and final idea that is causing the bad mpg? Would I be able to junkyard parts mod it back to normal. Or is there a possibility it is extremely well hidden?
I appreciate the help greatly!

[VStar650CL]

I wonder if Harry Backyard tacked it to the front of the radiator. There should only be two sensors there, not three. I don't think there was a crash zone sensor on your vehicle, that's the only other thing that might be up there. I'd try yanking the connectors with the scanner attached and see if one of them makes the reading plunge.

[mdmellott]

If my car is missing the IAT sensor, would you know where the OBD2 scanner is getting an IAT temp of 120*F from. I recall from past live scans of it being at 108*F, so I assume a fluctuation means something somewhere is giving the ECU that info. BUT
FROM WHERE? This is really frustrating because I feel I am at the LAST and final idea that is causing the bad mpg? Would I be able to junkyard parts mod it back to normal. Or is there a possibility it is extremely well hidden?
I appreciate the help greatly!

As much as I appreciate the Haynes manuals, sometimes it does throw you a curve ball. The picture showing the IAT sensor location near the left headlight for 3.3L V6 models is actually showing a Frontier. Your Pathfinder IAT is not located facing the backside of the left headlight, as shown on that Frontier. It's more like around the corner, facing the left fender, if I'm reading the dealership's drawing correctly. The following two links to Nissan Parts Deal show the '98 Pathfinder and '98 Frontier with 3.3L engines, so you can get a better idea where to look for your IAT. and where not to look.
https://www.nissanpartsdeal.com/parts-l ... G33E;5=SE)
https://www.nissanpartsdeal.com/parts-l ... m=165_B002

[VStar650CL]

The picture showing the IAT sensor location near the left headlight for 3.3L V6 models is actually showing a Frontier. Your Pathfinder IAT is not located facing the backside of the left headlight, as shown on that Frontier. It's more like around the corner, facing the left fender

Dunno what I'd do without you on these oldies, Md. I only see 'em once in a blue moon now, and you lose what you don't use. Tyvm!

[mdmellott]

Or is there a possibility it is extremely well hidden?

Now that I have looked a bit closer at your picture of the air cleaner box and compared that to the dealership drawing, I think the IAT is extremely well hidden. Like, inside the left fender.

[Alibireason]

My head started spinning. Ya'll are cracking the case on this and I am loving it. Thank you.

Has anyone taken the wheel well shroud off? I'm thinking I will be staring at a brick wall, but just maybe I will be staring right at it.
Removing the air filter housing, I believe, thinking from the comfort on my couch, will only lead to the inside of the tube the sensor is located.
Remove headlight??? Head is spinning. I am extremely grateful for all you people.

[mdmellott]

My head started spinning. Ya'll are cracking the case on this and I am loving it. Thank you.

Has anyone taken the wheel well shroud off? I'm thinking I will be staring at a brick wall, but just maybe I will be staring right at it.
Removing the air filter housing, I believe, thinking from the comfort on my couch, will only lead to the inside of the tube the sensor is located.
Remove headlight??? Head is spinning. I am extremely grateful for all you people.

It's mounted on a large tube attached to the air duct assembly box onto which the air filter box is mounted. Not sure if it's accessed behind the forward wheel well splash shield but I know it's hiding inside the forward portion of the fender area. That wasn't much help but I haven't had my hands on a '98. No headlight removal needed.

[Alibireason]

Went to junkyard today, for practice, found a 1997 Pathfinder, took the front wheel well splash guard off and was staring at the back side of the tube leading up to the air filter, reached hand around and felt the IAT sensor. I swiveled over to the front, and someone already had take the orange turn signal out and there it was. PITA to twist and pull the rubber out, but I finally got it out of the whole. Then it wouldn't release from the harness so I just pulled it out of the harness. The clamp on this beast would not budge so I sprayed PB Blaster and waited. Still not budging. Next I put some needle nose vice grips on, lightly putting pressure and I hear a pop and the clamp was now moving and the sensor came right out.

My new Hitatchi IAT charge sensor is in the mail. It'll be like Xmas morning as a kid when it gets here.

I took out my turn signal and tried to hand grip the harness to remove it and was seized like the junkyard one. When the new sensor arrives I'm hoping to just get the vice grips in there and remove the harness while the sensor is still in the tube.
In the picture the top is the overflow coolant tank for reference.

[Mike W.]

Neither one of those is IAT, the IAT sensor is integrated with the MAF. One is ambient temperature for the HVAC system, the other is ambient temperature for the display in the rearview.

I'd wondered what those were for. No issues, but I do like to know what's where and for what.

[Alibireason]

I got the sensor and installed it. Tested the old one with multimeter and heated water and it tested as working. Not a waste of time because you great folks led to me knowing how to actually find the sensor, and I'm sure it was pretty old (new Hitatchi one on RockAuto only $23).

I took a long drive and went to auto parts store to use obd2 scanner. With key in ignition but not turned on I had an IAT reading of 78*F. I was in the store about 10 min. Turn on the car and the IAT number just started climbing, reaching 124*F before I turned the car off. As I sat there with the car off the temp started dropping about 2*F every 15 seconds. It got down to 106*F before I returned scanner.

Temperature outside was 55*F.

With the IAT sensor working, what else could be causing the sensor temp to read this high compared to outside temp?
Speculating, but could it be a crack in the housing that leads up to or holds the sensor, letting in heat produced by the engine? My only thought based on zero experience is that the intake is bringing in air already heated by the hot car itself.
Any thoughts? Thank you>

[VStar650CL]

You're probably just getting heat-soak in all the mechanical parts after a 10 minute stop, including the body of the IAT sensor. If you measure your block temperature after a short stop using a laser thermometer you'll see the same thing, it will hit peak temperature about 15~20 minutes after shutoff. That's because no coolant is circulating and the internal heat has to go somewhere, so it heats up the air and everything else inside the engine box.

[Alibireason]

"You're probably just getting heat-soak in all the mechanical parts after a 10 minute stop, including the body of the IAT sensor. If you measure your block temperature after a short stop using a laser thermometer you'll see the same thing, it will hit peak temperature about 15~20 minutes after shutoff. That's because no coolant is circulating and the internal heat has to go somewhere, so it heats up the air and everything else inside the engine box."-VStar650CL

Not sure if I'm reading you correctly. Are you saying that since coolant is no longer a factor, that the temp of an engine will INCREASE after the engine has been stopped for a short period? My experience was that I drove for 30 minutes, let it rest for appx 10 minutes while obtaining scanner, and the temp, with engine off, after 10 min, was 78*F. The opposite of what you just stated.
It wasn't until I started car that temps started rising to WAY above outside temp of 55*F.

Also you stated earlier that my car is acting like it is in Alaska, with frigid temp IAT readings, when I stated the opposite is happening. My IAT is reading at over 120*F, like the Sahara desert. You have me very confused.

[VStar650CL]

Not sure if I'm reading you correctly. Are you saying that since coolant is no longer a factor, that the temp of an engine will INCREASE after the engine has been stopped for a short period? My experience was that I drove for 30 minutes, let it rest for appx 10 minutes while obtaining scanner, and the temp, with engine off, after 10 min, was 78*F. The opposite of what you just stated.
It wasn't until I started car that temps started rising to WAY above outside temp of 55*F.

You won't generally see the results of the heat soak in the dash gauge because of where the sensor is located, on the water outlet. If you run-up a high idle right after a soak you may see a blip in the gauge as the hot stuff left in the block starts to flow to the radiator, but that's all. However, you'll see it very clearly if you aim a thermometer at the block itself. The coolant will basically cook and the block temp will rise by as much as 30 degrees. The hot cats also dissipate heat when the engine is shut down, so all in all the engine room will be an awfully hot place about 15 minutes after shutdown.

[VStar650CL]

Also you stated earlier that my car is acting like it is in Alaska, with frigid temp IAT readings, when I stated the opposite is happening. My IAT is reading at over 120*F, like the Sahara desert.

PS - This just means something else is responsible for the crappy mileage, not the IAT. It sounds like your IAT is working about right.

[Alibireason]

VStar650CL, will you please explain then why, based on what I have stated earlier, my IAT numbers go down after I turn my car off as opposed to up, which is what you are claiming? It seems my car and obd2 scanner are going against your understanding of this issue. And please clarify your Alaska comparison from earlier, it seems to have no context on my mpg situation. Also please explain how my sensor is located on a "water outlet?"
-Thank you

[VStar650CL]

VStar650CL, will you please explain then why, based on what I have stated earlier, my IAT numbers go down after I turn my car off as opposed to up, which is what you are claiming? It seems my car and obd2 scanner are going against your understanding of this issue.

Sitting there idling after a long drive the temp will climb for the same reason, reduced coolant flow and the engine and cats shedding heat. If the IAT sensor isn't climbing after shutoff, it may just be because yours is buried in the fender and not in the box with the MAF like later models.

Also please explain how my sensor is located on a "water outlet?"

You're mixing apples and oranges. That referred to the ECT sensor and not the IAT.

[mdmellott]

VStar650CL, will you please explain then why, based on what I have stated earlier, my IAT numbers go down after I turn my car off as opposed to up, which is what you are claiming? It seems my car and obd2 scanner are going against your understanding of this issue. And please clarify your Alaska comparison from earlier, it seems to have no context on my mpg situation. Also please explain how my sensor is located on a "water outlet?"
-Thank you

You're chasing a gremlin that doesn't exist. Let it go. The reason you see the IAT sensor reading spike upwards when monitored by a scan tool, after the warmed up car has sat with the engine off and then started again, is because of all the heat of the engine and driver's side cat in particular has heated up the cavity inside the fender where the air intake snorkel inlet resides. The hot air is sucked up when you start your car and then the IAT sensor spikes. The "Alaska" reference was obviously stated in a hypothetical context in relation to a possible missing or dead open IAT contact that would be seen by a scan tool with a reading of -20F or -40F, hypothetically. The reason the temperature reading you see actually drops during this engine off but warmed up period is exactly for the reason you were told. The IAT is mounted away from the engine, on the air duct assembly, positioned low in the fender cavity where heat is not trapped like it is higher up where the snorkel inlet is at. Therefore, the temperature reading drops because it is not affected by the heat rising within the fender cavity.

[mdmellott]

When a person goes to the doctor for any reason at all, the first thing the doctor does is check for your basic vital signs. I find this analogous to beginning any unknown engine diagnostics. Using a scan tool to check the basic vital signs of the engine's operation can save you a great deal of time and unnecessary expense on parts that are not needed nor contributing to what is ailing your vehicle. Long term fuel trim (LTFT) is often a most useful piece of information. For example: If LTFT is grossly different on bank1 compared to bank2 then you can immediately rule out components that effect both banks equally. (i.e. IAT, MAF, ECT, fuel regulator, fuel pump, etc.) Using the scan tool to check the behavior of your upstream O2 sensors can save you time guessing what may be ailing your car if you know how to interpret what the data represents. Get a decent scan tool to use to help with diagnostics. If you take your car to a reputable service technician, they may ask you to bend over and cough up some money but they are likely to give you an answer to your poor mpg issue.

[Alibireason]

mdmellott thank you very much for clearing all of this up for me! "Bend over and cough up some money" has me cracking up. (clearing throat) I will do just that. Case closed then?

[VStar650CL]

Case closed then?

No, keep us posted when you find out what the mileage issue really is. Alaska is only one of many things that can cause it.

[Alibireason]

No, keep us posted when you find out what the mileage issue really is. Alaska is only one of many things that can cause it.

Will do. Thank you!

I just learned how to quote.