Bank 1 runs "lean" after upstream oxygen sensor replacement



I can hear some hissing from the top, not from the wheel well

gonna look for smog machine or just use propane?


If you hear a hissing you have likley found the problem.


I hope, sir! :slight_smile:

Unfortunately I could not confirm it with a carb-cleaner spray, just to be sure.
Gasket set is not very expensive, and by this point I can get all the top removed and reinstalled with closed eyes.

I wonder WHY it would leak?
Connected worry is if anything was cracked.

Sure, I was applying some brute force as I was working to loosen nuts on the catalyst, on this side sure enough, but could not imagine it might somehow pop up on the heads-to-intake connection??


Intake manifolds are prone to mate unevenly. There are various techniques to get the manifold to nestle into position evenly and only with close attention or a great deal of luck can the mating be successful. Even the small block Chevy intakes took some care to get settled squarely in place so as not to leak.


not very reassuring :frowning:

still, how happened it failed after 150K miles…

anyway, will post update once I’m there


I thought you had removed and replaced the upper plenum. The seal between the upper and lower sections would be the difficult joint.


I used “crazy Russian” method to make a smoke machine on a dime:

  • plastic bottle
  • hot-glued piece of hose to the cap
  • hot-glue compressor trigger-handle to the bottom
  • cigarette into the cap’s hole
  • lit it up and blow :slight_smile:

I had some smoke through the tail-pipe, but importantly, I have smoke from under the lower intake parts, so diagnosis is official at this point - somehow Ive got a hole there

anybody knows if Nissan’s VQ40DE (4.0L V6) has coolant circulating through the lower intake?

I managed to avoid draining cooling system so far, as I could take the plenum tilted and shuffled aside doing all other work, so ould we a great surprise to get coolant all around removing aluminum intake header


Who was it that said “the harder I work the luckier I get?”


I do not know, man, but I do really appreciate your and all others ideas and advice!


Please define “smoke from under the lower intake parts”

Do you mean smoke is coming out, from under the intake gaskets?

That would mean the intake gaskets themselves aren’t sealing . . . I thought you had replaced them?

Do you mean the manifold itself is leaking?


I replaced plenum/plastic-to-metal (upper) gaskets, not lower metal-to-metal ones


I just looked at a service manual online for a VQ series V6 . . . not sure if 4 liter, though

Apparently no coolant running through it

Where is the thermostat located . . . is it on the front of the engine, on the block, or on the timing case cover?


once I removed plenum and injectors, it became clear it has no coolant there


It sounds like you’ve got things under control there OP, good on you for for keeping on keeping on and not yielding. As I read through the thread, first thing I’m thinking an exhaust leak. Why? Lean on one side only. You’ve got two exhaust manifold interfaces with the block, and if bank 1 leaked there, that would create this symptom. The exhaust manifold/block interface gets really hot and bothered as a normal routine so the gasket could easily fail, or the fasteners could just work themselves loose from the hot/cold cycling. Exhaust manifold can crack from the heat too. I’m talking about those that hold the exhaust manifold to the block on that side. You might poke around in that area, check to see if the fasteners are – well — fastened.

The intake manifold/block interface was next in my mental list of suspicions, and it sounds like you may have a clue that in fact is the problem. Usually that would be easily proved with the starter or propane spray test, checking for increased idle rpm. But if that’s not the problem … hmmm …

Next in line is the power brake booster. Esp if it gets its vacuum source on the bank one side.

After that … leaking egr or malfunctioning pcv systems.

BTW, I presume you’ve done the normal vacuum system testing already right? Where you connect a hand held vacuum pump to each manifold vacuum source line and check that that vacuum line and everything it is connected to holds vacuum.


I spoke too soon

all gaskets on intake replaced now, still same reading on bank 1

checked leaks on EGR, brake booster: nothing there (disconnection test)

hiss is still there, but I’m not sure it is not air coming through the choke

loos like I bound to get access to good smoke machine instead of home-grown solutions


Have you tested the pcv valve? That’s a simple thing to check, and if it is slightly stuck more open than it should, that’s the same thing as a vacuum leak. I had that problem on my truck this past summer, it was idling a little lean. The pcv valve passed the clicking and suction tests, but it still seemed the truck’s ancient pcv valve might be passing air a little too freely. I soaked it in gasoline overnight, and that seemed to improve things measurably, but not completely. So I replaced it. That cured the lean idle problem.

Does your Nissan’s pcv system route air into bank 1? If so, definitely consider the pcv valve as a suspect.



I am having almost the same issue with my 05 4WD Pathfinder. I had a PO430 on B2. So far I have completed the following with no neg results found:
Intake vacuum leak ck by using propane 3 times, Scanner Danner water test 3 times, smoke test 3 times both at the intake and tailpipe, vacuum gauge test 3 times to include pinching off brake booster hose. New PCV, It only cost $7 new and it is in such a cramped access space I just R&R, old one checked good, exhaust shop vac test 2 times, engine running @ 2000 RPM and idle exhaust leak ck by hand, balance ck on B2 all 3 fuel injectors and all 3 coils, Nissan MX manual ck list for checking spark plugs and coils, fuel pressure test, late night with no lights open hood to inspect for voltage leaks (stray electric arching on wire harnesses), timing light ck, mechanic’s stethoscope ck, hold brake pedle down during engine start to see if pedle depresses more (if it does it may indicate a vacuum leak inside brake booster). Check AT fluid level, color, smell, small air/foam bubbles in fluid is a sign of a problem

My B1 AF Sensor stays @ 3v but swings up and down during engine rev, my B2 AF Sensor stays around 6v and also swings during rev, but unlike your code mine is on B2 with the AF S @ 6v and yours is on B1 with AF S @ 3v.

I took mine to an independent shop, they said I needed a new cat on B2, $800+

I declined, then swapped out AF S, no change in scan data on my blue driver scanner, AF S volts same as before.

My MPG is down by 2.5 from 17 (17 normal for 4WD Off Road Model) to 14.5.

My scanner data shows me indications of a lean condition on B2 and no indication of a fuel delivery issue. This lean condition would normally indicate vacuum leak, exhaust leak, exhaust blockage, bad injector, bad coil, bad spark plug, bad AF or O2 sensor, bad cat, continuous misfire, and a few more. But so far I have found no evidence of a cause. No other codes except PO430. If it is a bad cat, normally there should be a cause that made it fail? I do not want to R&R without trying to determine then fix the root issue also.

Before taking a 500+mile round trip to Phoenix from Vegas I change oil and filter, cleaned MAF and throttle body (first time to ever clean MAF and TB). I cleared the CEL code before leaving. On the drive I monitored my scanner, LTFT, STFT, AF, O2 looked normal, mpg still 14.5. The CEL came back on @ about 50 miles into the trip, I expected this, assuming the old PO430. I made a pit stop and noticed my RPM at idle was 950+ (it should be 625 + or - 50, which on my car it always has been until then). The engine seemed to be running fine despite what I assumed to be the PO430 CEL, I WOT a couple times on HWY 93 getting easily up 105 mph.

When I got to Phoenix I read the CEL code, expecting PO430, instead only had one current and pending PO507, no PO430.

Got Home 12 hrs later, still PO507, no PO430.

Researched PO507, Idle speed control system RPM higher than
expected. My Nissan MX manual says it is caused by either a vacuum leak, a bad pcv hose, or a bad ECM. Other web sources say it is more likely a bad TB unit. I also made the mistake of manually moving the TB butterfly without unplugging it first during cleaning, several web sites say doing this messes up the calibration?

I also found a Nissan Service Bulletin NTB05-067:

DTC P0507 (ISC SYSTEM) is stored,
• This incident may be the result of recent cleaning or replacement of the throttle body.
The vehicle “in-use” learned idle (IAVL) may be higher after throttle body deposits are
reduced (by cleaning or replacement).

It goes on to say you have to take it to the dealer or shop with high end scanner and a Tech with the know how to follow the procedure.

I will take it to a new shop that just opened close by.

That just might fix the idle, if not I will put out $130 for a rebuilt TB.

I noted that on top of my engine’s TB there is a small clear amount of epoxy like glue that covers what looks like a calibration adjustment nut. I am tempted to drill open the IAC on the side of drive by wire TB, I have seen this done on youtube, to inspect it for corrosion and or stripped gears.

My idle is now 1500 RPMs in Park, 900+ in Drive (load on engine) while fully stopped by brake, after several relearn attempts no change. Also while in idle after I rev the engine it will rev up and down on its own 1500 to 2500 RPMs some what slowly for 30 sec after I release the pedle. My MPG now reads 10.4.

I also suspect that the high idle RPMs might be causing a engine condition that now results in sensors not being able to detect the PO430 code. It is very likely once I get the PO507 code fixed my buddy PO430 will be back?

Maybe the off chance of cleaning the MAF may have fixed the PO430?

I will brief the shopTech on the whole situation when I take it in for the Nissan Service Bulletin PO507 procedure. In a way I wanted to figure it out myself, but I do not have access or the know how to operate advance scanners that is needed to complete the bulletin procedure.

Other things (from $0 to a lot $$$$, not in order) I was going to do to possibly fix PO430:

Do a B2 exhaust back pressure test (blockage unlikely because I already did several vacuum gauge tests, just because there is no blockage does not mean the cat is not bad, AF and or O2 will slam a PO420 or PO430 once cat becomes less efficient, but still partially works).

Ck muffler for blockage also.

Ck for Weak ignition component.

Weak AF or O2 sensor.

Complete the Nissan MX Manual PO430 ck list.

Complete the Walker Exhaust trouble diagnostic ck list.

Ck for bad engine or AT mounts.

Ck the B2 camshaft position sensor.

Test AF and O2 sensors.

Ck AT operating good. If AT is not working right it will effect engine performance.

B2 Cylinder Compression test, if head gasket has even a small coolant or oil leak into one or two cylinders it will cause a cat fail, also it could cause a spark plug to be fouled enough during initial start up (cold engine) that it will not fire for a few seconds until it starts working after the coolant or oil is vaporized and the engine warms up, in the mean time the cat is getting coated with fuel, oil and or coolant.

Some of the hardest trouble shooting is on systems that are only intermittently working, or at a working range where it only sometimes causes a code, Wire Harnesses too, frayed wires that are barely still connected and can stop or start working again because the car jostles around.


Clean or R&R fuel injectors

R&R O2



Take to shop

Take to Dealer

Another possible (unlikely) cause is loose battery cables, or one or more of the 4 grounds within the engine compartment are loose/corroded, two by the battery inside Rt front fender, two by the air filter inside Lt front fender.

I have read you can have many misfires before a code shows up, so if misfires occur only in certain times and stop occurring it may be enough to be the cause of another systems code being activated but not the misfire code.

There is suppose to be a way to see actual misfire totals using some scanners mode 6, even without a misfire code being activated.

Use IR temp gun to ck Cat temp after fully hot and engine running, should be 30% hotter just past the cat then in front, different results for different cars, I would only use this as an indication, not absolute evidence.

You can rent a block tester from Auto Zone, checks to see if coolant is leaking internally inside your block and heads.

Ck to see if your PF 06 requires ECM Re-flash (its like a software update).

Ck all recalls and Nissan Service Bulletins for your 06.

Do WOT test with scanner, ck out Scanner Danner how to vids on youtube.

Another Note:


Be advised that your 06 Pathfinder is highly probable to have a AT failure due to radiator coolant leakage into AT case from cross contamination, if you notice a milky substance in your radiator reservoir, that is AT fluid, that means rad fluid is in your AT. Do a web search on “Pathfinder AT failure” you can R&R radiator or do a mod for less $. A AT R&R is Thousands $$$$.

I will update my progress, but I may not be doing anymore work on my PF until after the New Year.

I look forward to your next update.



looks like we share this upstream sensor low voltage symptoms, but you went much further into both diagnostics and into the troubles developing around

so far, every step I took (like cleaning injectors and swapping left/right sides to see if problem migrates) only made engine to work better and better, but still low on bank-1 sensor

last step (replacing intake manifold gaskets) made it to whisper and idle absolutely even: no vibration at all

still, that low voltage on bank-1

I gave up at this point, as I have to drive long trip in a week, so I took it to the shop for diagnostics

they were a little bit blown by my story and amount of work I put into it so far, told great I eliminated virtually everytihg in upper side, so they will take it from here

waiting on results, hopefully today


By the way, I hear a hiss also and it is really loud now that my idle RPM is high, it is the air going through my TB (in my case anyway).

Also when I referred to AF as 3v and 6v it should have been 0.3V and 0.6V.

My PF MX book has spark plug torque at 24.5 ft lbs

If you find a low compression test result on a cylinder you can then perform a leak down test to find if it is caused by a Block problem or a head issue.

A vacuum gauge can tell you a lot about the condition of an engine:

For PF should be 17-21 Hg at idle.

@ 2000 RPM if it is lower than idle reading that would mean an exhaust restriction.

many other ways to ck engine with vacuum gauge, ck out vids on youtube.

also on PF brake booster where the hose goes into the booster that little black connector is a one way check valve, if your doing a smoke test, smoke will not be able to get past that check valve and you will assume your booster has no vacuum leak, best to use a vacuum gauge on a vacuum port on the intake, once you have a steady reading pinch off the hose to the booster with vice grips, if the vacuum reading increases (a higher Hg) that means you have a vacuum leak somewhere from the point the vise grip is on the hose to inside the booster.

I found another error in MX book, in another section it said PO507 could also be caused by Electric throttle control actuator (the all in one TB).

Although the MX book refers to an EGR in an example, there is no EGR on a 2005 Pathfinder, the fuel system is one way with evap for fumes, I believe this is also true for an 06.


MX book says AF sensor should operate around 1.5V. That will show on Offical Nissan scanners. These after market scanners use algorithms to approximate readings, some scanners have fine print saying readings can be off as much as 5X the real V.
We both have the same AF V rates, I believe our AF S are good.