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Discussion Starter #1
Hey folks,

Been diagnosing the AFR sensors in my 08 Nismo as I got a P2A00 code (Bank 1 Sensor 1 Low) at the shop.

I plugged in my scan tool (Innova 3040) and scrolled through the live view PID's until I found the two pre-cat sensors.
These are wideband sensors.. my understanding is that the voltage readings should be somewhere around 0.3v-0.7v... BUT my scanner was telling me that the two sensors were running at 2.0V and 2.1V. There's no way this is possible. Both lambda values seem fine, sitting at ~98-99%.
Am I mis-interpreting the data? Is my scan tool inaccurate for some reason? are both my sensors kaput?

Hope you guys can help!
 

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That does seem strange. Do you have a reading from the rear o2 sensors?
 

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Discussion Starter #3
Ok so I warmed up the truck, didn't take it for a drive just let it get up to temp in the driveway.

Grabbed the PID's off live, here they are with post-cat first:

02 B1 S2: 0.28v
-STFT B1 S2: 99.2%
02 B2 S2: 0.28v
-STFT B2 S2: 99.2%

Those seem normal.

lambda B1 S1: 0.994
02 B1 S1: 2.197v
lambda B2 S1: 0.995
02 B2 S1: 2.177v

I lack the experience reading these values - could this difference just indicate that the cats are doing their job?

Here's the fuel trim values for good measure
STFT B1: -4.7%
LTFT B1: 0.0
STFT B2: -2.3
LTFT B2: 0.0

Both cats were sitting at 183 deg F, but again, that's after only warming up for a few minutes.
 

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First, here's my internet waiver. I am by no means an expert, but I have done a little tuning on my own. Here are my thoughts.

"my understanding is that the voltage readings should be somewhere around 0.3v-0.7v"
This is mostly true. But understand that the voltage reacts to current conditions. If the engine is targeting the stoich value for gas (14.7air to 1gas) then ideally you should see about .500mv, if it is actually achieving stoich (14.7). But engines aren't perfect so some variation is allowed and an averaging method is used, ie. STFT(Short Term Fuel Trim) and LTFT(Long Term Fuel Trim).

Actually, your O2 values look very odd to me. They show a lean condition but the trims are pulling fuel (-2 and -4) as if there was a rich condition.

Some questions come to mind.

How is the truck running?
What have you done to it?
Recently?
Are you sure the precat sensors are wideband?
Has it been tuned by someone?

You may want to seek the advice of a professional. :)
 

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Discussion Starter #5
Thanks for the reply! Internet waiver understood and accepted.


I'm in a unique situation.. can't 'really' drive the thing right now because I'm trying to get it to pass a strict interprovincial inspection right now, as such it's not licensed. I drove it ~390 miles from the town I bought it in and it did the whole drive on a single tank, so I have some suspicion that it's running lean. It seems to idle ok, maybe a bit grumbley and around 700rpm.

All of this is part of diagnosing the P2A00 code. Interestingly, I've erased it for now by cleaning the MAF, changing the air filter and cleaning the throttle body. I understand MAF-equipped vehicles can run lean in a state where the sensor is dirty, and judging from the condition of the air filter and air intake I would not be surprised if that were the case. Despite the current lack of error code I'm still pursuing this AFR sensor idea in case there's something deeper going on..
That's all the work I've done so far, and I doubt the previous owner put a tune on.


As for the wideband sensors, my understanding is that vehicles past a certain model year (my '08 included) were all equipped with them.
Also, the fact that the ratios are presented as 'lambda' values is a give-away, I suppose.

So maybe I'll get out for a clandestine evening drive in the next couple of days, get the thing going a little bit and let the LTFT's give me some data.
 

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Cleaning everything may very well do the trick.

To isolate a "bad" sensor you could also swap the front sensors and see if the code moves to the other bank.

I don't suppose your scanner can save the entire drive log?
 

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Ok so I warmed up the truck, didn't take it for a drive just let it get up to temp in the driveway.

Grabbed the PID's off live, here they are with post-cat first:

02 B1 S2: 0.28v
-STFT B1 S2: 99.2%
02 B2 S2: 0.28v
-STFT B2 S2: 99.2%

Those seem normal.

lambda B1 S1: 0.994
02 B1 S1: 2.197v
lambda B2 S1: 0.995
02 B2 S1: 2.177v

I lack the experience reading these values - could this difference just indicate that the cats are doing their job?

Here's the fuel trim values for good measure
STFT B1: -4.7%
LTFT B1: 0.0
STFT B2: -2.3
LTFT B2: 0.0

Both cats were sitting at 183 deg F, but again, that's after only warming up for a few minutes.
No such thing as Short Term Down Stream B1S2-B2S2.. it's a bogus place holder. All you care about is the voltage range..So ignore the 99.2% reading..
 

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I might be out of date but when engine is cold your not getting accurate readings, 183 degrees is still a cold engine as far as cat temps go.
 

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Stoichiometric on Nissan is 2.200v. You should be checking with engine warmed up and at 2,000rpm.
 
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