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#4719619 - 04/07/18 03:35 PM Bad O2 or Cat? I have data to help determine.
Datarock Offline


Registered: 06/23/12
Posts: 47
Loc: San Antonio, TX
I have 2008 Nissan Versa Hatchback. The MIL has recently come on and it is showing a P0240 (Catalyst System Efficiency Below Threshold Bank 1).
I know it's sometimes hard to diagnose the correct reason for this code so I have provided some data that I hope can assist with pointing to the right component that is bad. I am thinking it's the bank one O2 Sensor but would like a second opinion.
Car runs great and I have not noticed any difference in MPG or performance.











Edited by Datarock (04/07/18 03:36 PM)

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#4719626 - 04/07/18 03:41 PM Re: Bad O2 or Cat? I have data to help determine. [Re: Datarock]
mk378 Online   content


Registered: 09/27/15
Posts: 1332
Loc: USA
You need to look at the time graph of both sensors. If they are both swinging at about the same rate, that means the sensors are OK but the cat is not doing much. If you see varying voltage from both sensors, a P0420 is most likely the cat. Also make sure there are no exhaust leaks.


Edited by mk378 (04/07/18 03:42 PM)

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#4719630 - 04/07/18 03:43 PM Re: Bad O2 or Cat? I have data to help determine. [Re: Datarock]
maxdustington Offline


Registered: 01/21/17
Posts: 234
Loc: Toronna
I had a car that would throw that code, I kept resetting it and eventually it went away.

I thought it was P0420, not P0240? Perhaps I am mistaken.

It could be the rear 02 or the cat. I think you need a scan tool to observe the signal from that rear 02 sensor to see if it is lazy or malfunctioning.
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#4719638 - 04/07/18 03:48 PM Re: Bad O2 or Cat? I have data to help determine. [Re: Datarock]
eljefino Offline


Registered: 06/15/03
Posts: 32384
Loc: ME
It seems weird that the O2 sensor diag is not a prerequisite for the cat. I'd definitely fix that first so its I/M can "click in".

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#4719686 - 04/07/18 04:44 PM Re: Bad O2 or Cat? I have data to help determine. [Re: Datarock]
Linctex Offline


Registered: 12/31/16
Posts: 5718
Loc: Waco, TX
I have never heard of an O2 sensor after the cat going bad, ever.
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#4719687 - 04/07/18 04:45 PM Re: Bad O2 or Cat? I have data to help determine. [Re: Datarock]
thastinger Offline


Registered: 10/23/17
Posts: 53
Loc: VA
can you log your short term fuel trims and O2 voltage? or at least see them? Those values should be changing constantly, if the O2 voltage never changes then it is bad

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#4719771 - 04/07/18 05:57 PM Re: Bad O2 or Cat? I have data to help determine. [Re: Datarock]
michaelluscher Offline


Registered: 10/07/12
Posts: 2382
Loc: Staten Island, NY
If you don't want to buy a cat, and emissions inspection in your area are lax, consider a new post cat O2 sensor, with a spacer attached

All of a sudden, it'll be 92% efficient again

Now, if it's broken apart internally, or clogged to the point where your down on power or overheating, track down the misfire that killed the old one, and replace it

Also consider using a better OBD app

This data layout gave me eye cancer, Torque or Torque pro is well worth the $3, then you could properly graph the O2 sensor voltage
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#4719772 - 04/07/18 05:57 PM Re: Bad O2 or Cat? I have data to help determine. [Re: Datarock]
Trav Offline


Registered: 11/20/06
Posts: 18257
Loc: MA, Mittelfranken.de
Whats the misfire and fuel fail? Check the misfire count.
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#4719848 - 04/07/18 07:19 PM Re: Bad O2 or Cat? I have data to help determine. [Re: Datarock]
Marco620 Offline


Registered: 02/25/14
Posts: 1969
Loc: Deplorable in KS/ fly over
How many miles on car? I had a weird smell before mine went out.
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#4719873 - 04/07/18 07:40 PM Re: Bad O2 or Cat? I have data to help determine. [Re: Linctex]
slacktide_bitog Offline


Registered: 03/20/08
Posts: 5853
Loc: USA
Bad oxygen sensors have their own separate codes.

Originally Posted By: Linctex
I have never heard of an O2 sensor after the cat going bad, ever.


I have

4th gen Maxima, got a code associated with the bank 1 sensor 2 sensor, which is the post-cat oxygen sensor. Don't remember the code, but I remember it translating to the sensor after the cat. I replaced that sensor, and the code never came back. Car has 280k now, had about 260k when the code came on

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#4719912 - 04/07/18 08:10 PM Re: Bad O2 or Cat? I have data to help determine. [Re: Datarock]
andyd Offline


Registered: 09/25/04
Posts: 6884
Loc: Marshfield , MA
Replace the failed O2 snsor. Cheaper and easier than the cat. A meter is constantly in motion. If it doest change in the blink of an eye between .2 and .9 V, change it.
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#4720023 - 04/07/18 09:55 PM Re: Bad O2 or Cat? I have data to help determine. [Re: andyd]
pdxglocker9mm Offline


Registered: 11/09/15
Posts: 75
Loc: Portland, OR
Graphs were one of the tools i used to diagnose bad cats.
I would overlay the pre and postcat sensor readings on the scan tool.
If the post cat matched the pre cat graphing basically 75% of the time that was a good indicator of bad cats. We called it “ghosting”
If the pre and post cat graphs were converse or basically opposite that ususally meant a good cat.
If the post cat or pre cat sensor was a flat line and or very slow to switch but you could see fuel trims rise and fall that usually meant bad sensors.

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