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Bank 1 runs "lean" after upstream oxygen sensor replacement

This is 4.0L V6 Nissan Pathfinder.
Originally it threw P0420 code, which is “Bank 1 Catalyst efficiency below normal”, so I replaced catalyst on Bank 1.
Next it threw PA200, which is for faulty upstream oxygen sensor on Bank 1.
I measured what old sensor was showing, and it was running in narrow range of plus/minus 0.1 volts around 0.26V.
It looked like a faulty sensor, so I replaced it, then ran “Relearn Air/Fuel Mix” procedure.
New sensor shows 0.30V during idle on Bank 1, while the one on Bank 2 idles around 0.60V.
It looked strange and as a bad replacement sensor, so I took car for a test drive and while breaking by the engine, both sensors were showing around 0.95V, so they both were able to detect “rich” condition very nicely.
Still, Bank 1 would return to 0.30V at idle, which is definitely “lean”.

Any ideas how to “gently push” engine’s ECU to get to the normal range?

Check for a vacuum leak on the lean side.


Vacuum lines are shared between Bank 1 and Bank 2 actually.

I thought about exhaust, but as I was putting a replacement cat there, I had it rock-solid: had to squeeze that gasket for a couple of millimeters

The only other place I can imagine is an intake manifold: it has individual pipes, so it might be the only one there.

Red_Knox: thanks for a hint… now I recall I was taking intake plenum off to replace spark plugs and I trusted the guy on YouTube who was saying that plenum gaskets are reusable… maybe, but not after 10 years of service probably… getting my gasket set tomorrow and investigating this avenue!

Did you install an OEM sensor. I have had problems when not using OEM O2 sensors.

Fault codes do NOT mean “This component is bad. Replace it.”

They are merely one part of your diagnosis. They are like a tool. You have to know how to use them correctly.

You have in essence shot the messenger, versus performing proper diagnosis

There’s a very good chance neither of those were in fact the problem

I’m with Rod Knox . . . but use a smoke machine to check for leaks. Even an exhaust leak ahead of the oxygen sensor is a leak, which the sensor will pick up. Don’t limit yourself to only looking for intake manifold leaks

And with today’s technology, you could have a good sized leak, which you will not hear, and the engine will run just fine, but it’s enough to cause lean codes and turn on the mil

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So, intake plenum gaskets are brand new now, still getting around 0.30V readings at idle or on constant load, still full swing reaches 0.95V.

I’m not sure if downstream sensor[s] might be anyhow connected to ECU keeping it on lean side.

What are the trim numbers and does the O2 sweep above and below .5v when the rpm is held at 2,000?

I’ve reset ECU keeping battery disconnected overnight.

At first, I see long-term trims to be at 0%, so it was indeed reset.
Short-term trims are +8.5% on both sides.

Using “Olivia Drive” Android app to talk to my ELM327 as the only one where I could find dynamic sensor parameters to be reported, but looks like the developer substracts 100% from the “classical value” reads.

that was idle

gonna re-check for 2000 rpm now

Not sure if it applies to your car, but I had bad o-rings on the plenum causing the issue of bank running lean on a Ford Windstar.

Just replaced that o-rings, but thanks for the suggestion

Short term trims on 2000 rpm:

Bank 1 - flustiating 4.6 to 7, averaging arund 6%
Bank 2 - fluctuating -1 to +4, averages around 0

Upstream sensors behave exactly the same on 2000 rpm:

bank1 - averages 0.29V
bank 2 - averages 0.6V

I’ve checks for VISUAL leaks between the engine block and upstream sensor: no obvious ones

based on 2000 rpm test: I can not imagine it to be a leak on exhaust: it would not have anough vacuum to exploit it on these rpms

both bank 1 and bank 2 are flipping through 0.5 V: I have to rev engine and let it go: it will show well above 0.5V when RPMs decrease (and one would expect to have a rich reading)

posted results (hit wrong “reply” button)

I assume you asked for 2000 rpm test to get an indirect check on vacuum leaks in exhaust manifolds, didn’t you?

I find it very confusing: how it can be held at 0.3V flip-point on bank-1 so stubbornly and independent of RPMs or load?

On another side, it kinda plays into the argument that it is ECU what decides to keep it at 0.3V at any cost, since sensor is glady going into 0.95V when pushed.

Not sure if you like to live dangerously, so I refuse to support spraying starting fluid on areas to note change in rpm to pinpoint a leak. Don’t try this at home.

“But we are going to do it anyway!” Sunday non? religious song.

What instrument are you using to measure the voltage from the O2 sensors?


ELM327 reading for dynamic parameters: 100mS sampling rate

Out of desperation: swapped ignition coils between banks to check if problem follows colis: no change

Have no idea if it is anyhow connected to injectors?
Tested their resitance: all test around 13-14 ohms.

Are the O2 sensors identical? Can they be switched?

FINALLY, I’m getting somewhere :slight_smile:

I decided to remove spark plugs: to check if any cylinder would be somehow different (trying to find: maybe it is a plug itself, maybe it’s injector)

the very first plug was barely held on last thread in the well !?!?!

I’m not daydreaming guys, I’ve torqued them to spec and then 1 Nm atop of the spek when I was installing them, checked twice… and now it is off like that ??

Can we get some more information, please?

Are you saying that the plug was loose?

And if so, are you saying that you torqued them to spec, and it still was loose afterwards?

Is it possible you somehow crossthreaded the plug when you were installing it?

Please don’t take that as an insult . . . it’s been known to happen. For instance, when you get rushed, distracted, etc.

Any chance the wrong plug somehow got installed?

Did you by chance drown the spark plug threads in antiseize paste? If so, that will make your torque readings inaccurate, and could lead to buggering up the threads

Without seeing any pictures, it’s possible the threads in the cylinder head are damaged. You may be able to restore them with a special thread chaser, which is readily available at your local parts store. If that doesn’t work, you may need to install a helicoil, timesert, etc.