I removed the lower and upper intakes on my 1997 Pathfinder. Changed the fuel injectors (hitachi), new IACV, Knock Sensor (Hitachi), fuel rail seals to lower intake, upper and lower intake gaskets (Nissan), 90 degree coolant hose at back of intake, 90 degree coolant hose on the thermostat housing to lower intake, cleaned insides of upper and lower intakes with spray cleaner, and egr gasket.

Vehicle runs nice and smooth now and is much more responsive than before. However, I got a code the next day (P0156) for the Bank 2 Sensor 2 oxygen sensor. I did not have this code before the job.

I had a Bank 1 Sensor 1 code a few months ago and I replaced that sensor and that code never came back. But I'm wondering before I rush to change this sensor if its related to the work I did.

For the heck of it I ran through some live data and I see the SHRTFT1 is between 15% and 17%. The SHRTFT2 is between 13% and 14%. And these trims were actually running about 2 percent higher yesterday but have came down today. The long term fuel trims are still at 0% and I assume because its only been about 1.5 days since the computer was reset. Had the battery off for about 3 days.

I tried spraying around at various places in search of a possible vacuum leak and I can find none. Most vac lines were also replaced. The very few that were not are in good condition.

Again absolutely no driveability, hesistation, or power issues highway or local and no odd idling. 750rpms in drive and around 800 to 850rpm steady in park. Are these fuel trims and oxygen sensor related? Could it be all the cleaner from cleaning the intakes and lubing the o rings on the injectors with oil that are burning off?

I did everything properly with no rush and checked every twice. I dont see where a possible issue could arise from the job. Maybe just a coincidence.

A1218 wrote: ↑

Tue Mar 23, 2021 5:46 pm

Are these fuel trims and oxygen sensor related? Could it be all the cleaner from cleaning the intakes and lubing the o rings on the injectors with oil that are burning off?

The downstream O2 sensors (sensor 2) have no affect to engine performance at all. Those only monitor the effectiveness of your catalytic converters. The STFT are a bit on the high side if those readings were at idle. However, until the LTFT's are set by the ECM, you won't have the total picture. Ideally, if LTFT is set at zero by the ECM, the vehicle if performing perfectly to where the factory has set the nominal fuel injector pulse width modulation. Any deviation from zero is a percentage measurement above or below the nominal pulse width. The STFT constantly changes. The LTFT is set after the STFT is recognized by the ECM as a stable average number. If you take the Pathfinder for a steady drive on the freeway, the LTFT should get set, ideally very close to zero. By now any cleaners you used are all burned off. Once set, if the LTFT on both banks are at around +15%, you have a lean condition issue going on and your ECM is compensating by opening the injectors wider to add more fuel to the air/fuel mix. Once set, the LTFT % becomes the new STFT zero point and any deviation from that is above or below the set LTFT reading. Both of these fuel trim percentages change frequently but LTFT lags behind STFT. STFT changes every time you press or let off the accelerator pedal and can be very high numbers when you floor it.

mdmellott wrote: ↑

Tue Mar 23, 2021 6:40 pm

A1218 wrote: ↑

Tue Mar 23, 2021 5:46 pm

Are these fuel trims and oxygen sensor related? Could it be all the cleaner from cleaning the intakes and lubing the o rings on the injectors with oil that are burning off?

The downstream O2 sensors (sensor 2) have no affect to engine performance at all. Those only monitor the effectiveness of your catalytic converters. The STFT are a bit on the high side if those readings were at idle. However, until the LTFT's are set by the ECM, you won't have the total picture. Ideally, if LTFT is set at zero by the ECM, the vehicle if performing perfectly to where the factory has set the nominal fuel injector pulse width modulation. Any deviation from zero is a percentage measurement above or below the nominal pulse width. The STFT constantly changes. The LTFT is set after the STFT is recognized by the ECM as a stable average number. If you take the Pathfinder for a steady drive on the freeway, the LTFT should get set, ideally very close to zero. By now any cleaners you used are all burned off. Once set, if the LTFT on both banks are at around +15%, you have lean condition issue going on and your ECM is compensating by opening the injectors wider to add more fuel to the air/fuel mix.

I’ll check back the trims after a good highway drive.

And yeah I read in the manual that those do not affect engine operation. But I also saw that it can trip this code from vacuum leaks?

A1218 wrote: ↑

Tue Mar 23, 2021 6:55 pm

But I also saw that it can trip this code from vacuum leaks?

Not sure if that's possible but if your scanner is capable of looking at your O2 sensors and you do have a lean condition caused by a vacuum leak, you will see the O2 sensor 1 readings look flattened out at the low voltage end of its voltage modulation. The sensor 1 voltage modulations are constant at around .1V to .9V and can be seen on an oscilloscope, or a scanner with a live data graph feature, as a typical sinusoidal wave. If the bottom of the wave is more of a wide tough than a steep valley, that's an indication of a vacuum leak. Looking at those readings, you should have the engine rev'd a bit more than idle.

(Regarding my previous statement, when I mentioned the ECM compensating by opening the injectors wider, I meant to say longer not wider. And as for the time it takes for the LTFT to get set by the ECM, that should happen rather quickly. Within a few minutes, if not seconds, if I'm not mistaken.)

mdmellott wrote: ↑

Tue Mar 23, 2021 7:22 pm

A1218 wrote: ↑

Tue Mar 23, 2021 6:55 pm

But I also saw that it can trip this code from vacuum leaks?

Not sure if that's possible but if your scanner is capable of looking at your O2 sensors and you do have a lean condition caused by a vacuum leak, you will see the O2 sensor 1 readings look flattened out at the low voltage end of its voltage modulation. The sensor 1 voltage modulations are constant at around .1V to .9V and can be seen on an oscilloscope, or a scanner with a live data graph feature, as a typical sinusoidal wave. If the bottom of the wave is more of a wide tough than a steep valley, that's an indication of a vacuum leak.

Yeah my scanner has that feature. I’ll check it out tomorrow.

I tried spraying around with carb/throttle spray, but didn’t come up with anything.

But if there was a vac leak, aside from fuel trim wouldn’t I see some noticeable affect on the engine operation itself? Maybe a fluctuating or high idle or even rough idle?

A1218 wrote: ↑

Tue Mar 23, 2021 8:09 pm

I tried spraying around with carb/throttle spray, but didn’t come up with anything.

But if there was a vac leak, aside from fuel trim wouldn’t I see some noticeable affect on the engine operation itself? Maybe a fluctuating or high idle or even rough idle?

Yeah ... I would think so. Give it a shot (I assume your using brake cleaner) down around the lower intake manifold gasket as well as around the upper intake manifold gasket. If you still get no reaction, you're probably fine. If it is a very small leak, where ever it is, if there is one, spraying brake cleaner may not result in your engine revving higher if only an insufficient amount is sucked up. However, you should be able to see an effect to the O2 sensor 1 if even a small amount of vapor is sucked up. The top end of the voltage peak will flatten out if a vacuum leak is sucking up the vapor.

As for the O2 sensor 2 fault code; Maybe it has gone south on you. The scanner should see those O2 sensors as well. The voltage on those should always be relatively stable. They do not modulate like the upstream sensors. If it's flatlined, it's a dead sensor or the circuit to it is open like in a broken wire, corroded connector contacts, or not plugged in well.

One more note on using brake cleaner in this application of it: It is not kind to plastic parts like connectors on your injectors and elsewhere, plastic components, rubber and vinyl seals, and hoses. For this reason, I prefer to use a propane torch. You can point the torch nozzle (not lit of course) right at suspect areas and find even the smallest of leaks while observing the O2 S1 sensor readings on a scanner. You can't reach lower intake manifold gasket areas very well but you can douse the general area well enough for this method to be effective. Obviously, extreme caution should be observed to not create a propane filled engine compartment. Do just one area at a time and then let the gas vapors dissipate before moving on to check other suspect points where a vacuum leak might be.

mdmellott wrote: ↑

Tue Mar 23, 2021 9:18 pm

One more note on using brake cleaner in this application of it: It is not kind to plastic parts like connectors on your injectors and elsewhere, plastic components, rubber and vinyl seals, and hoses. For this reason, I prefer to use a propane torch. You can point the torch nozzle (not lit of course) right at suspect areas and find even the smallest of leaks while observing the O2 S1 sensor readings on a scanner. You can't reach lower intake manifold gasket areas very well but you can douse the general area well enough for this method to be effective. Obviously, extreme caution should be observed to not create a propane filled engine compartment. Do just one area at a time and then let the gas vapors dissipate before moving on to check other suspect points where a vacuum leak might be.

So I made some adjustments. I checked and adjusted the tps to 0.56v which is within spec. I adjusted the IACV with the brown tps plug removed. It was a little out of adjustment as I replaced it the other day. It’s idling around 800rpms in park and 750 drive.

Checked the fuel trims and the short term 1 is around 5% now and the short term 2 is around 1%. Long terms are now set and around 8%.

And the O2 code is now gone.

I started this thread a few months ago after getting a P0156 O2 sensor code following a refresh of engine components when I pulled the upper plenum and lower intake.

After putting the vehicle back together I set the idle a bit higher than spec, around 850rpms and the code showed up. I dropped the idle down to 750 and reset the computer. The code never came back.

Two weeks I decided to raise the idle back up again out of curiosity and the code showed back up shortly after. So I dropped it back to 750rpms again and it never went away for about a week. I decided to reset the computer and I’m waiting for all the monitors to ready in order to see if it will come back.

Is this normal to occur? I read the manual and a bank 2 sensor 2 O2 code can be triggered by a vacuum leak as well. Is the computer seeing the raised rpms from opening the idle screw as a leak?

I have emissions inspection next month so in case I have to change it, it would be great to do it in advance. I had this sensor changed once by a friend at his shop when I was on a trip up in Canada and turns out he welded it onto the pipe. So this wouldn’t be as simple now.

A1218 wrote: ↑

Mon Aug 23, 2021 4:47 pm

I have emissions inspection next month so in case I have to change it, it would be great to do it in advance. I had this sensor changed once by a friend at his shop when I was on a trip up in Canada and turns out he welded it onto the pipe. So this wouldn’t be as simple now.

Probably the threads got trashed getting the old one out, that's pretty common. There usually isn't enough metal left for a heli-coil (plus thread kits that size are hideously expensive). So welding or brazing was probably the only way to save the pipe.

ECU's from that era have incomplete control of the engine functions, they're relying on certain things to be set properly by humans. Idle speed is one of them, so yes, it's entirely possible an elevated idle was making the ECU draw incorrect conclusions about the mixture and combustion.

Makes sense. I’ll see what happens once the monitor goes into ready state and analyzes the system.