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Discussion Starter · #1 ·
Year: 2007
Engine: 4.0L V6
Mileage: 93,000

My service engine light recently came on, a bit after I noticed that my truck was idling erratically. So I pulled out my code reader and got a P0171 code (System too lean bank 1). After some further exploration, my initial thoughts are a faulty upstream O2 sensor. My readings at idle (after car was driven for 3 hours) are as follows:

Bank 1, Sensor 1: 0.475v
Bank 1, Sensor 2: 0.15v
Bank 2, Sensor 1: 0.9v
Bank 2, Sensor 2: 0.15v

So from these readings, it appears that I may have a faulty Bank 2, Sensor 1 (oddly not bank 1). Would folks agree with that? From reading other posts, I gather that it could still be an injector issue, but I would think that if Bank 1 upstream is reading 0.475, it likely isn't fuel pressure or mass air flow (?). My mass air flow readings ranged from 0.01 to 0.04(ish) lb/s as I increased rpm's.

My next question is, assuming that Bank 2, Sensor 1 is, in fact, faulty, why would both of the downstream sensors be reading so lean? I would expect that if the upstream sensor was faulty and reading too rich, the computer would lean out the mix due to the faulty upstream reading, which would then cause the downstream sensor (which I assume to be working correctly) to read lean. But why would BOTH downstream sensors be reading lean when the Bank 1 upstream sensor is reading a healthy 0.475?

I guess another possibility is that both downstream sensors went out...but that seems unlikely at the same time.

I brought the engine up to around 4,000 rpm a number of times and saw both downstream sensors jump to around 0.85, and they were both reading approximately the same value. At those higher rpm's, the upstream sensors didn't change much from their idle values (there were spikes and troughs in the plots, but all within about 0.1v of the idle baseline).

Also, note that I don't know that much about these O2 sensor problems and diagnostics...only what I've been able to understand from previous posts.
 

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Discussion Starter · #2 ·
Update:

Replaced both upstream O2 sensors. Readings are still the same. Idle was still erratic until I erased the code, at which point the idle became normal. But readings are still the same, so I find that odd. What would be causing one side to be running super rich? Maybe Bank 2 injectors? Or an air leak somewhere? I don't hear the engine mis-firing, so I don't think it is bad spark plugs (plus I replaced them all 30k miles ago).

Unless anyone has another suggestion, I guess my next step is to try and check the injectors. Tough though, as the intake system completely covers the passenger side injectors (and spark plugs), so it's quite a task just to expose them.
 

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Update:

Replaced both upstream O2 sensors. Readings are still the same. Idle was still erratic until I erased the code, at which point the idle became normal. But readings are still the same, so I find that odd. What would be causing one side to be running super rich? Maybe Bank 2 injectors? Or an air leak somewhere? I don't hear the engine mis-firing, so I don't think it is bad spark plugs (plus I replaced them all 30k miles ago).

Unless anyone has another suggestion, I guess my next step is to try and check the injectors. Tough though, as the intake system completely covers the passenger side injectors (and spark plugs), so it's quite a task just to expose them.
Make sure all your basics are covered before throwing money at parts. When I bought my 01 with 99k on the clock, it was showing a code for an 02 sensor. I took it to autozone with all intentions of buying the sensor. The parts guy told me to check all my vacum lines and the fuel filter. He said a restricted fuel filter could cause problems such as low fuel pressure which would tell the sensor I was running lean. I replaced the fuel filter ( which looked original) and after about 100 miles, the light went off by itself. Just my experience, good luck.
 

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Discussion Starter · #4 ·
Thanks for the tip. I was staring to look at vacuum lines next. Could be fuel filter, but I replaced that when I had to replace the fuel pump assembly about 20k miles ago. Still easy to check and replace, so I'll give that a shot. The vacuum leak is a tough one since there are so many vacuum lines concealed by the intake manifold. Dont think I can run and test for leaks with the intake manifold removed. But I can at least visually inspect them. The reason why I initially suspected O2 sensor was because bank 1 was reading normal while bank 2 was reading high. Figured if I had low fuel pressure from pump or filter, it would affect both banks. Could certainly be wrong on that, though.
 

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Thanks for the tip. I was staring to look at vacuum lines next. Could be fuel filter, but I replaced that when I had to replace the fuel pump assembly about 20k miles ago. Still easy to check and replace, so I'll give that a shot. The vacuum leak is a tough one since there are so many vacuum lines concealed by the intake manifold. Dont think I can run and test for leaks with the intake manifold removed. But I can at least visually inspect them. The reason why I initially suspected O2 sensor was because bank 1 was reading normal while bank 2 was reading high. Figured if I had low fuel pressure from pump or filter, it would affect both banks. Could certainly be wrong on that, though.
Sorry I cant be of more help, but I think you are on the right track. Hopefully some other members may have some advice for you. Please keep us updated to keep this thread fresh. Sometimes its a little quiet here on the weekends. But I am sure someone else may be able to put you on the right track.
 

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Discussion Starter · #6 ·
Just remembered that 2007 frontier does not have external fuel filter...it is integral to the pump assembly, which I had to replace not too long ago (as mentioned). Why Nissan did that is beyond me.
 

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Just remembered that 2007 frontier does not have external fuel filter...it is integral to the pump assembly, which I had to replace not too long ago (as mentioned). Why Nissan did that is beyond me.
Not sure which ones but there is a recall for 2nd gen fuel pumps. Go to nissans recall website and put in your vin#.
 

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Discussion Starter · #8 ·
No recall on mine. Pulled off the intake manifold and cleaned and inspected all the seals and vacuum hoses I could find. Put back together, same readings. I do feel like I'm hearing a slight hiss coming from somewhere, but it doesnt appear to be any of the hoses (cant rule them out, though). Is it possible that I could have a cracked exhaust manifold? Since the problem is only affecting bank 2, is it possible that there is a crack in the bank 2 exhaust upstream of the O2 sensor, letting O2 out causing the sensor to read rich? It is nearly impossible to get a good visual of the exhaust manifold, so I'm just spitballing right now. Just trying to come up with something that would only affect bank 2.
 

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No recall on mine. Pulled off the intake manifold and cleaned and inspected all the seals and vacuum hoses I could find. Put back together, same readings. I do feel like I'm hearing a slight hiss coming from somewhere, but it doesnt appear to be any of the hoses (cant rule them out, though). Is it possible that I could have a cracked exhaust manifold? Since the problem is only affecting bank 2, is it possible that there is a crack in the bank 2 exhaust upstream of the O2 sensor, letting O2 out causing the sensor to read rich? It is nearly impossible to get a good visual of the exhaust manifold, so I'm just spitballing right now. Just trying to come up with something that would only affect bank 2.
You might be on to something there. I know the 1st gen was notorious for cracked manifolds, I dont know if it was fixed before the 2nd gens came out. Have you tried using the search on the site? There is so much info on here if you do a little research.
 

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Thanks for the tip. I was staring to look at vacuum lines next. Could be fuel filter, but I replaced that when I had to replace the fuel pump assembly about 20k miles ago. Still easy to check and replace, so I'll give that a shot. The vacuum leak is a tough one since there are so many vacuum lines concealed by the intake manifold. Dont think I can run and test for leaks with the intake manifold removed. But I can at least visually inspect them. The reason why I initially suspected O2 sensor was because bank 1 was reading normal while bank 2 was reading high. Figured if I had low fuel pressure from pump or filter, it would affect both banks. Could certainly be wrong on that, though.
I had the same situation. 140K, 2010 SE. After digging around a found a cracked vacuum hose. They tend to rip right at the connection point to a plastic nipple.
 

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I just fixed this code (P0171) on mine, truck did not seemed to have any misfires, just the code coming back every 20-50 miles after erasing it. The heater core junction ("T") on the outlet side was slowing and finely leaking where no water was seen but if you would touch it, you could feel it wet around. It did have a very fine and small crack that slowly build droplets that later would fall over passenger side Oxygen sensor, thus throwing the code P0171, I guess the sudden change in temperature from coolant hitting the sensor and/or exhaust manifold would trigger a "lean mixture" reading. I ordered a brass T and got a hose and clamps from local auto parts store to fix it, did the heater core inlet piece as well with the Dorman aftermarket part and added coolant lost. About 135 miles after fix and code hasn't come back when last week it was becoming more frequent popping up after 8-15 miles after light reset.
 

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2007 Frontier 87K miles. I just got this code on the way home on the highway. I'm leaning towards the brake booster hose as it happened when I had to stop quickly. Also, my STFT on B1 according to my Cortex tuner is 0.0 while the LTFT are 20's and STFT b2 is under 6 at idle. If I press the brake hard and release STFT B1 will jump to 20's for a split second after releasing. Should I do the starter fluid trick around hoses and such?
 

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P0171 has been back, I might try what @Total nutcase recommended, Truck does not miss, if I erase code, it comes back with no consistency in the mileage, one time it returned after 8 miles, the other after 300... I have sparkplugs that I'll be replacing since it needs them anyway at 105K miles, bought with 103k, 2005 LE.
 

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The readings will change when the brakes are applied because that affects intake vacuum. I wouldn’t worry about that too much. Most likely you got something else wrong
But why would bank 1 stft be 0.0 and bank 2 be 6.2 at idle?
 

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Most likely there is a vacuum leak somewhere, but because of the brake booster line location I doubt it would affect one bank more than the other. But then again, the VIAS system might change that, but I wouldn’t know which bank if thats the case
 
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