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Bad O2 or Cat? I have data to help determine. #4719619
04/07/18 02:35 PM
04/07/18 02:35 PM
Joined: Jun 2012
Posts: 67
San Antonio, TX
Datarock Offline OP
Datarock  Offline OP
Joined: Jun 2012
Posts: 67
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.










Last edited by Datarock; 04/07/18 02:36 PM.

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Re: Bad O2 or Cat? I have data to help determine. [Re: Datarock] #4719626
04/07/18 02:41 PM
04/07/18 02:41 PM
Joined: Sep 2015
Posts: 1,595
USA
mk378 Offline
mk378  Offline
Joined: Sep 2015
Posts: 1,595
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.

Last edited by mk378; 04/07/18 02:42 PM.
Re: Bad O2 or Cat? I have data to help determine. [Re: Datarock] #4719630
04/07/18 02:43 PM
04/07/18 02:43 PM
Joined: Jan 2017
Posts: 1,994
VW Fanboy Island
maxdustington Offline
maxdustington  Offline
Joined: Jan 2017
Posts: 1,994
VW Fanboy Island
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.


03 Jetta AWP/09A 205k kms
Edge 0W40 + Mann 719/30
Re: Bad O2 or Cat? I have data to help determine. [Re: Datarock] #4719638
04/07/18 02:48 PM
04/07/18 02:48 PM
Joined: Jun 2003
Posts: 33,564
ME
eljefino Online content
eljefino  Online Content
Joined: Jun 2003
Posts: 33,564
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".

Re: Bad O2 or Cat? I have data to help determine. [Re: Datarock] #4719686
04/07/18 03:44 PM
04/07/18 03:44 PM
Joined: Dec 2016
Posts: 7,151
Waco, TX
Linctex Offline
Linctex  Offline
Joined: Dec 2016
Posts: 7,151
Waco, TX
I have never heard of an O2 sensor after the cat going bad, ever.


"The evidence demands a verdict".
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Re: Bad O2 or Cat? I have data to help determine. [Re: Datarock] #4719687
04/07/18 03:45 PM
04/07/18 03:45 PM
Joined: Oct 2017
Posts: 87
VA
thastinger Offline
thastinger  Offline
Joined: Oct 2017
Posts: 87
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

Re: Bad O2 or Cat? I have data to help determine. [Re: Datarock] #4719771
04/07/18 04:57 PM
04/07/18 04:57 PM
Joined: Oct 2012
Posts: 2,475
Staten Island, NY
michaelluscher Offline
michaelluscher  Offline
Joined: Oct 2012
Posts: 2,475
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


'06 Toyota Camry SE V6 3MZ-FE 100k
'01 Mercury Villager Estate VG33E 52k
'06 Ford Taurus SE Vulcan 60k
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Re: Bad O2 or Cat? I have data to help determine. [Re: Datarock] #4719772
04/07/18 04:57 PM
04/07/18 04:57 PM
Joined: Nov 2006
Posts: 20,577
MA, Mittelfranken.de
Trav Offline
Trav  Offline
Joined: Nov 2006
Posts: 20,577
MA, Mittelfranken.de
Whats the misfire and fuel fail? Check the misfire count.


ASE L1, Master. Deutsch Meisterbrief.
Re: Bad O2 or Cat? I have data to help determine. [Re: Datarock] #4719848
04/07/18 06:19 PM
04/07/18 06:19 PM
Joined: Feb 2014
Posts: 2,449
[email protected] CZ firearm factory
Marco620 Offline
Marco620  Offline
Joined: Feb 2014
Posts: 2,449
[email protected] CZ firearm factory
How many miles on car? I had a weird smell before mine went out.


15' Civic R18 200,090k 0w20 Redline/Archoil9200/TEIN/Eibach/Injen/Tanabe Sustec/Borla Exhaust!
Going for 238,900 milestone. My house to moon is 238,900 miles!
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Re: Bad O2 or Cat? I have data to help determine. [Re: Linctex] #4719873
04/07/18 06:40 PM
04/07/18 06:40 PM
Joined: Mar 2008
Posts: 7,331
USA
slacktide_bitog Offline
slacktide_bitog  Offline
Joined: Mar 2008
Posts: 7,331
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

Re: Bad O2 or Cat? I have data to help determine. [Re: Datarock] #4719912
04/07/18 07:10 PM
04/07/18 07:10 PM
Joined: Sep 2004
Posts: 7,817
Marshfield , MA
andyd Online content
andyd  Online Content
Joined: Sep 2004
Posts: 7,817
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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'16 Camry SE, Valvoline HM 0w20 and OEM filter
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Re: Bad O2 or Cat? I have data to help determine. [Re: andyd] #4720023
04/07/18 08:55 PM
04/07/18 08:55 PM
Joined: Nov 2015
Posts: 75
Portland, OR
pdxglocker9mm Offline
pdxglocker9mm  Offline
Joined: Nov 2015
Posts: 75
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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