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afomenko 12-10-2016 03:20 PM

2006 Pathfinder: ECU runs lean on Bank 1 after replacing upstream oxygen sensor
Originally engine was throwing P0420 (Bank 1, catalyst performance below normal).

Replaced catalyst in Bank 1.

In 200 miles I've got PA200: same Bank 1, this time an upstream sensor seems to need replacement.
Shortly after I used "engine cleaner" fuel additive and thought it is a root cause (thread:, but looks like this is a wrong path.

Replaced upstream sensor in Bank 1, performed A/F mixture relearning.

Checking dynamic voltage: it will stay around 0.30V at idle (why?!?)

At first I suspected sensor is at fault, but when I run with breaking by engine (artificially creating rich condition): it jumps up to 0.90V, so sensor itslef is good.

On Bank 2, I have steady 0.60V at idle, 0.1..0.95V for the full range under the same engine-breaking test.

For some reason, ECU decides to feed Bank 1 lean, and Bank 2 slightly rich.

Previous sensor was "stuck" around 0.26V, barely moved anywhere from this voltage.
Looks like ECU "got used" to these low values and does not want to go up.

Any ideas?

afomenko 12-11-2016 07:50 PM

discussing it here:

so far I'm getting to the "faulty injector" conclusion

mpe235 12-11-2016 10:23 PM

Check the front exhaust pipe where it bolts to the bottom of the catalytic converter. I have seen several with cracks in the weld that cause the sensors to read erratic. Also with a scan tool you can clear the self learned values for the sensor.

afomenko 12-12-2016 06:52 AM

I made a "shop-vac plus soapy water spay" test and found a leak making for a pea-sized bubble to appear in 10-15 seconds, and YES, it is on the weld.

In my view this is way too small of the leak to account for such a big voltage drop, especially on high RPMs and under load, where NO WAY IMAGINABLE to have any vacuum in the system

afomenko 12-13-2016 03:55 PM

update here: after eliminating all odds, I have an officially confirmed leak on lower intake manifold, likely on gasket

see cartalk thread for details:

anybody knows if I can avoid draining coolant when removing intake manifold (lower, aluminum section) ?

afomenko 12-13-2016 03:59 PM

leak there looks to be quite "logical" for all my symptoms: this leak if more or less proportional to RPMs

In the same time let me make an advertisement to XADO Cermet Motor treatment I used on many old cars: I have compression around 13.0 atm on all cylinders after the fact, unfortunately did not take reading before treatment, but for my old subaru I've added 1-1.5 atm to compression, also to almost new shape

afomenko 01-07-2017 07:31 PM

if anybody cares where it went

voltage reported via OBD2 proved to be a red herring, as some normally running vehicles with no codes reported have exactly same values reported

I've completely eliminated all and any things in intake side: plugges small leak in intake manifold on gaskets, cleaned injectors, swapped ignition coils between banks and jonjectors between banks - it all made car to run smoother, but P2A00 was reappearing after some time

finally, I found leaks in catalyst I've placed when dealing with P0420, it made me think I'm on right track, then removing catalyst, I found that welding on the flange to downstream catalyst leaked substantially, broke off as I was removing catalyst

no mystery anymore, replacing both catalysts on passenger side and likely this is the end of story

smj999smj 01-08-2017 02:31 PM

Thanks for the update!

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