Bank 1 runs "lean" after upstream oxygen sensor replacement



Air is entering from somewhere. Lift the air cleaner lid from the air filter and block the air flow with a piece of plastic or duct tape. Does the engine stall? This will show if the air is entering through the throttle body or from a leak.


Thanks, I will do that in AM, will update


You’ve got a good point to try getting some other PF owner to read values from “known running good” car, so the basis can be established


It’s air, not O2 I think. IIRC the air to fuel ratio for complete combustion is about 14.7 pounds of air to 1 pound of fuel. Air is 20% O2 by weight as I remember. You can calculate the air to fuel ratio above yourself, if want to have a go at it. See if your calculations agree that it is air, not O2.

The chemical reaction for gasoline ignition in air is approximately

CH2 +(3/2) O2 = H20 + CO2

The relative weight of these atoms are

C = 12
O = 16
H = 1

Don’t forget to compensate for the amount of O2 in air, 20%.

Yes, that sounds correct to me. Whenever you have a high idle rpm it’s best to focus on fixing that first however. The reason is that’s going to be easier to fix, and whatever fixes that may fix all the rest of the problems. Probable causes of high idle rpm are

  • Coolant temp too low
  • Throttle valve misbehaving
  • Air leak bypassing throttle valve, directly to the engine, check
    • vacuum system
    • throttle valve bushings worn out, creating air leak
    • pcv system
    • intake manifold and cylinder head gaskets
    • any functions that purposely cause the idle rpm to increase to prevent stalling, like on many cars that would be turning the steering wheel, turning on the headlights, the rear window defrosters
    • egr system


I tried what you suggested, I did cover up my air intake while @ 1500 RPM, blocking it about 80% caused the engine to stall, did this several times, I would slowly cover the intake while listening for any leaks, as soon as I got to around 80% covered it would start to stall and if I did not start removing the blockage fast the engine would stop running.


Maybe your car’s computer is purposely keeping the idle speed higher than it should be for some reason. Try turning the steering wheel, turning on the headlights, effects of the engine radiator cooling fan turning on, the AC on, stuff that puts add’l load on the engine. How does doing those things affect the idle speed? On my Corolla I definitely notice the idle rpm increases if I turn on the headlights or turn the steering wheel while the engine is warm idling.


Yes will fix PO507 first, after trouble shooting intake leaks, PCV, I went to the Nissan forums, many posts about PO507, will try a few more no cost and self done procedures that worked for others, two of the most popular fixes though where taking it to dealer to have it relearned by the Nissan diagnostic machine or R&R TB.


Then I wait for my buddy PO430 to visit me again, I will R&R B2 fwd Cat with a Walker, did it myself 3 years ago on B1 with a Walker Cat, no issues since.

While looking through Nissan PF forums saw a lot of bad radiators causing AT to go bad, this happened to me at 98K, I had Geico MX Ins up until 100k, got lucky they paid full cost.

Noticed many other trends with PF, some I have had to fix others that have not yet occurred, like U-joints and of course the issue with high idle after cleaning TB.

I will now be a regular on these forums to keep myself informed on all issues 05 PF, and will review MX techniques and tricks before doing any non-routine fixes.


Nissan has a Service Bulletin out about PO507:

Classification: Reference: Date:
EC05-009 NTB05-067 October 5, 2005
APPLIED VEHICLES: All 2002 – 2005 models
The idle, in Park or Neutral, is more than 25 rpm above specification (with the engine
warm), and/or
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).
• If DTCs other than P0507 are stored, you must correct the cause for those DTCs first.
After correcting other DTCs, if you still have a P0507 and/or an idle condition above
specification, continue with this bulletin.
To reduce idle RPM, perform the following in this order until IAVL successfully completes:

  1. Check engine for vacuum leaks.
  2. Perform IAVL procedure.
  3. For 2002 and 2003 models: Check for newer ECM reprogramming data. If it’s available,
    perform reprogramming, then try IAVL again. 2004 and 2005 models: Skip this step.
  4. Reduce idle speed manually:
    a. Disconnect injectors one at a time until IAVL completes (see step 4 of the Service
    Procedure for details),
    b. Reconnect the injectors, clear DTCs, and perform final IAVL.

It goes on for several more pages, I have tried disconnecting injectors twice, will try a few more times.

But in the end it says if this fails you have to take it to dealer for fix.


Yes my idle will go down to 925 RPM in D while fully stopped by holding brake, so when there is a load on engine idle is reduced, but normal idle is 625 + or - 50, plus I can feel the AT shifting improperly while driving, and sometimes in park the idle will flux 1000 to 2400 RPM. These are common symptoms for Nissan’s with a PO507 code on the NISSAN forums.

Thanks for the help.


fyi, here’s a good explanation of the NPF automatic transmission problem.

"around 100,000 miles, the transmission cooler tank may rupture and coolant enters the transmission. Coolant in the transmission fluid causes almost immediate transmission failure, with no warning. A side problem is automatic transmission fluid may also contaminate the cooling system. "


Merry Christmas

After doing more research on the P0430 cause, and after all the trouble shooting done that ruled out many other causes to what could be the cause of the B2 CAT code, the current probable culprit is highly likely to be a leaking B2 fuel injector, lets see if my diagnoses is correct, I will have to pull the fuel tube out and pressure test it with the injectors still connected.

Will update if I was right or wrong on this one, if wrong the next step is to do some multimeter wire Voltage checks. If right maybe I caught it in time to where the CAT has not lost its efficiency and it is still good (unlikely).

I did find a tidbit of info that might give meaning to the AF Sensor Voltage readings (B1 0.3V and B2 0.6V) we have been getting on our after market scan tools? This is from the PF MX book DTC P0420/430 ck list:

Overall Function Check UBS00K8W
Use this procedure to check the overall function of the three way catalyst (manifold). During this check, a 1st
trip DTC might not be confirmed.

  1. Start engine and warm it up to the normal operating temperature.
  2. Turn ignition switch OFF and wait at least 10 seconds.
  3. Start engine and keep the engine speed between 3,500 and 4,000 rpm for at least 1 minute under no load.
  4. Let engine idle for 1 minute.
  5. Open engine hood.
  6. Set voltmeter probe between ECM terminals 74 [HO2S2 (bank
  1. signal], 55 [HO2S2 (bank 2) signal] and ground.
  1. Keep engine speed at 2,500 rpm constant under no load.
  2. Make sure that the voltage does not vary for more than 5 seconds.
    If the voltage fluctuation cycle takes less than 5 seconds, go to
    EC-316, “Diagnostic Procedure” .
    1 cycle: 0.6 - 1.0 → 0 - 0.3 → 0.6 - 1.0

The cycle above starts at 0.6V to 1V then cycles to 0V to 0.3V then back to 0.6V to 1V. Is it possible B1 AF Sensor has its base set to the second cycle 0.3V rate while B2 is set to 0.6V cycle the same as with the O2 Sensors above? It would make sense that the Banks would be on different cycles from each other while the scan tool reads the live data, if you think about it the scan tool reads some of the two Bank’s data as if there are two separate engines.

If anyone who reads this has a 05 or 06 Pathfinder and a scan tool what is your baseline Air Fuel Sensor Voltage readings on Bank 1 and Bank 2?

We have ours at B1 around 0.3V and B2 around 0.6V.



I thought you had swapped the fuel injectors, bank to bank. To see if the problem followed the fuel injectors. No?


that was the OP


Just found this, A 3rd PF with the same AF Sensor Voltage for B1 0.3V and B2 0.6V:

Joined: 14 Oct 2015
Posts: 4
Location: Edmond, Oklahoma

PostPosted: Wed Oct 14, 2015 3:04 pm Post subject: Reply with quote
I need some advice. I have a 2005 Pathfinder with about 185,000 miles. I have the P0430 code, but I do not think it is my cat. My reasoning is based on my O2 sensor readings. If I am correct, sensor 1 is before the first cat and sensor 2 is after. With the car engine off or at idle, sensors 1 and 2 on bank 1 both read 0.3 V. On bank 2, sensor 2 also reads 0.3 V, but sensor 1 reads 0.6 V. Similarly, as I drive the vehicle, sensor 1 on bank 2 always reads much higher than all the other sensors (and it is the only sensor that exceeds a reading of 1V). The post cat sensors on both banks always have similar numbers. Am I correct to assume my pre-cat sensor is faulty?

Unfortunately no one responded to his question.


Another one showing B1 and B2 at 0.3 and 0.6 volts, This is a Nissan Frontier:


Name of vid:




This one thinks the B1 @ 0.3V is the bad AFS?


this poor guy above got his banks mixed up and changed out AFS on the wrong bank twice thinking the new AFS was defective.