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Hello,

So I'm trying to diagnose what's causing my lean condition and code. I'm hoping someone here can run a quick check for me if they have the same vehicle as I do. I am currently checking the MAF sensor and while the engine was at idle I unplugged the wiring clip to the sensor. I've heard that the engine should kill itself but I've also heard that that doesn't apply to all vehicles. Could someone run this quick check and report back if that's the case?

The engine continued to run but like shit, obviously. I let it go for maybe 5 seconds without the MAF before turning the key.

Info:
Vehicle- 2009 Xterra X
Milage- 142k

Already tapped into the brake booster to check the vacuum and it's in the green but on the high side of green when idling. See picture.

I replaced both intake manifold tube between MAF and throttle body, I found small cracks in the underside of those. I also replaced an O2 sensor on the passenger side toward front right tire. Idk why but the updraft / downdraft terms confuse me with the O2 sensor diagram.

Any additional tips would be appreciated as I continue to work through the other potential issues for P0171. Thanks!
 

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Your only getting a Lean condition on one bank - so its likely something that only applies to that bank. If it were upstream of the manifold split you would be getting a lean code on both banks, or at least both long term fuel trims would show lean on both sides. Is that the case?

Its most likely your primary Air/Fuel sensor aka 02 sensor. But it could also be a manifold leak after the split or a injector stuck closed on that side. I think P0171 is bank 1 - which on our rigs is passenger side. If your replacing the sensor - get a Denso (which are OEM), or maybe Bosch.
 

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I had a lean condition twice....

1.) Replacing exhaust cat, the connector was loose which caused the car to report lean condition.
2.) After replacing a cat, the O2 sensor was bad. I soaked it in gasoline (advice from someone) which gave a slightly more accurate result, but finally I replaced the O2 sensor and it reported normal. 0.455V in live data.

Gonna share all I know about O2 sensors and debugging them below....

The voltage range for an O2 sensor is 0 to 1.0 V (once warm) - at an air/fuel ratio of 14.7 to 1, the sensor will read around 0.45 volts. (rich is high voltage, lean is low)

Keep in mind the O2 sensor generates its OWN voltage from the internal reaction (weird huh?), and then flows through a "bulb" that bulb compares the inside temperature to the outside temperature and reacts to the leftover oxygen in the exhaust. If the "bulb" or the Oxygen sensor is covered with any particulate, grease, exhaust dust, etc it can report lean. (which is why sometimes soaking them in gasoline will slightly fix them)

If you are getting a lean condition and your exhaust smells rich after warming the vehicle, you are almost guaranteed to have a bad O2 sensor.
O2 sensors are designed on the Xterra to last around 90k miles.

If you have a multimeter you can verify the actual voltage of the sensor (at the sensor) - you risk burning yourself against the exhaust so be careful - All O2 sensors (to my knowledge) are calibrated to produce 0.5 volts at "ideal" air/fuel ratio.
If the computer in the car thinks that the data coming from 1 of 4 O2 sensors in the Xterra is "ba" it often will put that bank in "open loop" or you'll see the car switching in and out of "open loop" periodically.
It is normal while driving down the road when you coast (or close the throttle while rolling) for the computer to go into "Open Loop" -The computer has no useful data to calculate air/fuel ratio from with a closed throttle, so it just uses pre-programmed "defaults" so it doesn't blow your engine up with a lean condition.

More reading:
The O2 sensor works like a miniature generator and produces its own voltage when it gets hot. Inside the vented cover on the end of the sensor that screws into the exhaust manifold is a zirconium ceramic bulb. The bulb is coated on the outside with a porous layer of platinum. Inside the bulb are two strips of platinum that serve as electrodes or contacts.

The outside of the bulb is exposed to the hot gases in the exhaust while the inside of the bulb is vented internally through the sensor body to the outside atmosphere. Older style oxygen sensors actually have a small hole in the body shell so air can enter the sensor, but newer style O2 sensors "breathe" through their wire connectors and have no vent hole. It's hard to believe, but the tiny amount of space between the insulation and wire provides enough room for air to seep into the sensor (for this reason, grease should never be used on O2 sensor connectors because it can block the flow of air). Venting the sensor through the wires rather than with a hole in the body reduces the risk of dirt or water contamination that could foul the sensor from the inside and cause it to fail. The difference in oxygen levels between the exhaust and outside air within the sensor causes voltage to flow through the ceramic bulb. The greater the difference, the higher the voltage reading.

Good Luck!
 

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Thanks for the info. That's pretty helpful. Makes me want to go soak mine in gasoline. I had the P0171 for a while and never found a conclusive cause. It's been gone for 3 weeks now but I'm sure it'll pop up again.
 

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Good reading. Thanks for posting. When I look at the live data on my 2005, Bank 1 Sensor 1 reads a pretty consistent .3 volts and Bank 2 Sensor 1 reads a pretty consistent .6 volts. I've read elsewhere that this is normal and expected. It's always been a mystery to me why they would be different values. I've also read (as you mentioned) that the ideal is .5, but I've never seen that so I'm still a little confused.
 

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Check for a small exhaust leak at both ends of the upstream cat. I had that happen to me after swapping in new cats I had a small ass leak on the outlet side of the up stream cat I ended up getting a multi layered steel gasket and putting high temp RTV between all the layers to finally get it to seal up.
 

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I'm new to exhaust leaks. What's the best way to track them down?
fill a spray bottle with about 1/3 dish soap and 2/3 water. Spray your exhaust around all the joints and start it up cold - the soap solution will blow bubbles where the leak is at.

You need to do it cold - hot exhaust will just evaporate the liquid on contact. You have about 2 minutes to find your leak then you get to wait an try again.
 

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Gonna share all I know about O2 sensors and debugging them below....

The voltage range for an O2 sensor is 0 to 1.0 V (once warm) - at an air/fuel ratio of 14.7 to 1, the sensor will read around 0.45 volts. (rich is high voltage, lean is low)

Keep in mind the O2 sensor generates its OWN voltage from the internal reaction (weird huh?), and then flows through a "bulb" that bulb compares the inside temperature to the outside temperature and reacts to the leftover oxygen in the exhaust. If the "bulb" or the Oxygen sensor is covered with any particulate, grease, exhaust dust, etc it can report lean. (which is why sometimes soaking them in gasoline will slightly fix them)

If you are getting a lean condition and your exhaust smells rich after warming the vehicle, you are almost guaranteed to have a bad O2 sensor.
O2 sensors are designed on the Xterra to last around 90k miles.

If you have a multimeter you can verify the actual voltage of the sensor (at the sensor) - you risk burning yourself against the exhaust so be careful - All O2 sensors (to my knowledge) are calibrated to produce 0.5 volts at "ideal" air/fuel ratio.
If the computer in the car thinks that the data coming from 1 of 4 O2 sensors in the Xterra is "ba" it often will put that bank in "open loop" or you'll see the car switching in and out of "open loop" periodically.
It is normal while driving down the road when you coast (or close the throttle while rolling) for the computer to go into "Open Loop" -The computer has no useful data to calculate air/fuel ratio from with a closed throttle, so it just uses pre-programmed "defaults" so it doesn't blow your engine up with a lean condition.

More reading:
The O2 sensor works like a miniature generator and produces its own voltage when it gets hot. Inside the vented cover on the end of the sensor that screws into the exhaust manifold is a zirconium ceramic bulb. The bulb is coated on the outside with a porous layer of platinum. Inside the bulb are two strips of platinum that serve as electrodes or contacts.

The outside of the bulb is exposed to the hot gases in the exhaust while the inside of the bulb is vented internally through the sensor body to the outside atmosphere. Older style oxygen sensors actually have a small hole in the body shell so air can enter the sensor, but newer style O2 sensors "breathe" through their wire connectors and have no vent hole. It's hard to believe, but the tiny amount of space between the insulation and wire provides enough room for air to seep into the sensor (for this reason, grease should never be used on O2 sensor connectors because it can block the flow of air). Venting the sensor through the wires rather than with a hole in the body reduces the risk of dirt or water contamination that could foul the sensor from the inside and cause it to fail. The difference in oxygen levels between the exhaust and outside air within the sensor causes voltage to flow through the ceramic bulb. The greater the difference, the higher the voltage reading.

Good Luck!
This is really good information - however the upstream sensors on I believe 2007+ (I may have that exact year wrong) are A/F sensors - not 02 sensors. My very limited understanding of A/F sensors is the voltage is simply a reference voltage from the ECU, the ECU actually gets its measurement in current - mA or micro amps or who knows, and converts it to a equivalence ratio (lambda) with 1.0 being ideal.

Like I said above I have found little if any useful info on A/F sensors practical troubleshooting - so if you know whats up with them any info you have would be helpful.
 

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Might have fixed my lean condition. I've had the P0171 for a few months now. I've replaced a/f sensors, MAF, had a buddy smoke test it (passed) and I've been ready to jump the gun on replacing cat(s) and the rest of the sensors. But said buddy (mechanic who's owned 23409245 Xterras already) said there's an issue with the airbox. The lower half of the box tends to bow out where the 3 loops that hold the top half in position. The center of mine was sticking out about 1/8" or so. I noticed I had some dirt on the clean side of the filter and clean side housing. Cleaned up the dirt, replaced the air filter, cleaned the MAF and used a heat gun to reshape the lower box so the 1/8" gap would be closed. After clearing the code, I went for a drive. It idles lower and smoother, my bank 1 fuel trims aren't jumping all over the place and so far, the code hasn't returned. Granted, it's only been a day, but it was to the point of returning on the first drive.

Fingers crossed that closing up that gap and cleaning the MAF is all I needed this whole time...
 

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Good reading. Thanks for posting. When I look at the live data on my 2005, Bank 1 Sensor 1 reads a pretty consistent .3 volts and Bank 2 Sensor 1 reads a pretty consistent .6 volts. I've read elsewhere that this is normal and expected. It's always been a mystery to me why they would be different values. I've also read (as you mentioned) that the ideal is .5, but I've never seen that so I'm still a little confused.
Same thing on my ‘06. It is def odd that those readings are considered normal for the pre a/f sensor years but based on the fact I’ve seen numerous people with rigs with no issues reporting those exact same values I’ll just say it must be right.
 

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Might have fixed my lean condition. I've had the P0171 for a few months now. I've replaced a/f sensors, MAF, had a buddy smoke test it (passed) and I've been ready to jump the gun on replacing cat(s) and the rest of the sensors. But said buddy (mechanic who's owned 23409245 Xterras already) said there's an issue with the airbox. The lower half of the box tends to bow out where the 3 loops that hold the top half in position. The center of mine was sticking out about 1/8" or so. I noticed I had some dirt on the clean side of the filter and clean side housing. Cleaned up the dirt, replaced the air filter, cleaned the MAF and used a heat gun to reshape the lower box so the 1/8" gap would be closed. After clearing the code, I went for a drive. It idles lower and smoother, my bank 1 fuel trims aren't jumping all over the place and so far, the code hasn't returned. Granted, it's only been a day, but it was to the point of returning on the first drive.

Fingers crossed that closing up that gap and cleaning the MAF is all I needed this whole time...
Fingers crossed for sure. I will say, though, that it would be a little odd if that was the cause because the air box is before both banks so an issue there would affect all cylinders. Theoretically speaking at least. Unless only one bank reports the lean condition due to the fact that, like previously mentioned, the upstream sensors on the older models read differing voltage values. So maybe they’re both affected but maybe the one that’s .3 changes to a value outside of the expected threshold. Either way interesting and I hope you actually did solve it permanently. Let us know if the code returns.
 

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Might have fixed my lean condition. I've had the P0171 for a few months now. I've replaced a/f sensors, MAF, had a buddy smoke test it (passed) and I've been ready to jump the gun on replacing cat(s) and the rest of the sensors. But said buddy (mechanic who's owned 23409245 Xterras already) said there's an issue with the airbox. The lower half of the box tends to bow out where the 3 loops that hold the top half in position. The center of mine was sticking out about 1/8" or so. I noticed I had some dirt on the clean side of the filter and clean side housing. Cleaned up the dirt, replaced the air filter, cleaned the MAF and used a heat gun to reshape the lower box so the 1/8" gap would be closed. After clearing the code, I went for a drive. It idles lower and smoother, my bank 1 fuel trims aren't jumping all over the place and so far, the code hasn't returned. Granted, it's only been a day, but it was to the point of returning on the first drive.

Fingers crossed that closing up that gap and cleaning the MAF is all I needed this whole time...
Yes, I hope its resolved, but I have trouble believing this because the MAF sensor is still downstream of this, and its the first thing that should have any affect. You technically could delete your air box and not get a lean code - although you would destroy your engine in a little while running unfiltered air.
 

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You know, I totally get the whole thing about why just bank one if it's coming from the airbox or MAF. It baffles me also because theoretically, it should throw codes for both banks. I've never gotten a straight answer when asking mechanics as to why a single source that splits and then is metered on each leg can throw a code for one side. Nobody knows.

So far, it's still running great, idling smoothly around 750rpm with no codes. The longest I ran without the code popping up (after it triggered the first time) was about 1 month. I'll keep an eye on the airbox bend and hopefully, post 3 months later that I'm still code free...
 

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Well, it came back last night. How about a leaky intake manifold? Bad gasket on the bank 1 side?
I am too lazy to re-read this thread. Did you change the primary A/F sensor. I have had 3 of these total , rich, lean and I can't remember. All 3 were primary A/F sensor, including one denso that was less than 6 months old.
 

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What does it mean when short term and long term are all over the place? Bank 1 is erratic and bank 2 runs pretty solid. Both sensors are new. As is MAF. No vacuum leaks.

Exhaust leak at or around the cat?
 
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