Post catalyst fuel trim DTCs, P209x

Randy Analyst Denver, Colorado Posted   Latest   Edited  
Demonstration
Emissions
2011 Dodge Dakota SLT 3.7L (EKG) 4-spd—1D7RE3GK0BS656035
P2098 - Post Catalyst Fuel Trim System Too Lean Bank 2

This vehicle came to me after a DIY screw up, then dealership fix/diagnosis. For the record, the dealer did everything right. It's a long and unclear story not worth going into. I was simply asked to confirm diagnosis after the dealer fixed the DIY "repair", which I did (same diagnosis as dealer).

It's the ultimate silver bullet for post cat fuel trim DTCs but I am sharing something I do all the time for these and for any other feedback issues. I plug the exhaust while watching O2 voltage, fuel trim and Lambda. This takes less than 15 minutes to either confirm or deny the common cause, exhaust leak.

Concern: Failed OBD emission test, P2098 commanding MIL.

Cause: Exhaust leak.

Correction: Replace both catalytic converters.

The pictures should explain everything.

diag​.​net/file/f2e4x0v8j…

diag​.​net/file/f42d1cl2k…

diag​.​net/file/f7bi40jev…

diag​.​net/file/f19lhajhh…

diag​.​net/file/f38k3flvk…

diag​.​net/file/f6kxvbazn…

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Interesting
Thanks
Nate Owner
Santa Cruz, California
Nate Default
 

Randy, What are the results of this test if there is not an exhaust leak?

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Randy Analyst
Denver, Colorado
Randy Default
 

Based on my scan data, I would have gone for cylinder imbalance next.

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Harm Technician
Fergus, Ontario
Harm Default
 

correction - welderup.

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Randy Analyst
Denver, Colorado
Randy Default
 

What about the pending DTCs? Cat readiness flag must be complete to pass in Colorado.

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Benjamin Technician
East Haven, Connecticut
Benjamin Default
 

Great writeup, sir.

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Rudy Technician
Montebello, California
Rudy Default
 

Not bad,for an "analyst"...... Ha!

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Randy Analyst
Denver, Colorado
Randy Default
 

Analyst was as close as I could find for job description. Any given week I will have a dozen different hats on. The last couple weeks it would have been mostly network engineer.:)

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Albin Diagnostician
Leavenworth, Washington
Albin Default
 

Randy, thanks for the rite up. I like the way you use the K.I.S.S. approach to the testing.

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Randy Analyst
Denver, Colorado
Randy Default
 

Always. It's because I spent 30 years as a lazy flat rater.:) I was thinking about you Sunday as I was felling trees, dragging them down with a winch and not having the proper logging equipment. I thought "I bet Albin could give me a few tips on speeding up this process.":)

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Albin Diagnostician
Leavenworth, Washington
Albin Default
 

Now we are equal :)

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Phil Technician
Tewksbury, Massachusetts
Phil Default
 

Would I be correct in assuming that you also had an audible although possibly hard to hear exhaust noise? It never ceases to amaze me how many times shops/techs ignore exhaust noises and then seem to struggle with readiness monitors not setting or codes for trim or o2 sensors and they don't put two and two together.

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Randy Analyst
Denver, Colorado
Randy Default
 

Hey Phil,.how's the NE OBD world? :) I could not hear the leak, even after diagnosis I revved the engine a few times without any audible exhaust noise. I use the rag in the tailpipe technique almost daily for the reasons you mention. It's so quick and accurate.

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Rudy Technician
Montebello, California
Rudy Default
 

Randy- Do you know if the dealer used the same technique, or was it more of a pattern failure thing for them? I admit I had not thought of using scan data to check for this issue, I would've gone the cave man route and pulled out the smoke machine.

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Randy Analyst
Denver, Colorado
Randy Default
 

I am guessing they did it visually since B2S1 is new.:) Yeh, I read about all the tests you guys do for these and I would say "stick a rag in the tailpipe and watch the O2 sensors along with lambda". No one seems to know what I mean so I posted this. I haven't put smoke up a tailpipe in over 10 years.

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Albin Diagnostician
Leavenworth, Washington
Albin Default
 

Randy, I knew exactly what you were talking about and doing. It seems I have heard that stuff several times,,,,,, from YOU along the way. :)

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Phil Technician
Tewksbury, Massachusetts
Phil Default
 

Things are good here, thanks for asking. Seeing the same things I always see here :-). Lots of folks with broken cars and shops that struggle to fix them sometimes. I hate to say it but most of the vehicle's I see that have monitor issues have something fairly obvious causing the issue, or at least contributing to the issue. At least with an MIL and a code eventually the parts shotgun usually…

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Randy Analyst
Denver, Colorado
Randy Default
 

It's no communication issues for me, I have 6 scheduled right now along with all my other duties. Last week it was K-line, this week it's CAN. Crazy busy, I think my training days are in the rear view mirror. I still love live vehicle presentations and helping technicians one on one but after 20 years of power point, I'm kind of over it for a while. I guess it's better than the old, click…

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Phil Technician
Tewksbury, Massachusetts
Phil Default
 

Thanks for the offer of assistance. I have to say that as for no communication issues, probably 80-90% or possibly more are simply pin fitment issues. Ford and Mazda are most common, but I think I have seen bad DLCs on just about every vehicle make at this point. I don't know why guys are so resistant to checking them. Everyone seems to get stuck on the fact that other scanners will communicate…

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Randy Analyst
Denver, Colorado
Randy Default
 

It's only one of two issues, hardware or software.:) I just had one kick my butt. Took me 1 hour and 10 minutes to solve it. That's a long time for a no communication diagnosis. It was software, I found it with a can sniffer. I don't think any other tool would have found this one. If I get time, I will post it but I have no idea how to display hex data rolling across a screen here. Pin fitment…

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Aaron Diagnostician
Council Bluffs, Iowa
Aaron Default
 

Interresting, but I really wish you would have shown B1. Do you know why an exhaust leak would force the ho2s high (representing a "rich" condition)? I am having a hard time making sense of it. It would seem plausible that plugging the exhaust may force the ho2s high, but that would also show on B1. I do believe you that it didn't change, just would like to see it myself, so I could see what a…

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Randy Analyst
Denver, Colorado
Randy Default
 

Albin explained it below. At low exhaust flows the exhaust pulses which draws oxygen in which then causes the O2 sensor to go low resulting in a rich command if being used by the ECM. When I plug the exhaust, no oxygen can come in through the leak so the O2 sensor only sees actual engine out exhaust which is rich due to the correction caused by the leak. There is a slight fuel trim adjustment…

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Aaron Diagnostician
Council Bluffs, Iowa
Aaron Default
 

Thank you for the more detailed explanation, I think I understand now. I do think the "normal" FT combined with the high o2s is what threw me off the most, but I did suspect it was due to no rear trim PID, as you confirmed.

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Steve Instructor
Irvine, California
Steve Default
 

This is great explanation of why you could have two opposing O2 sensor readings in the same exhaust system. Thank you for helping us.

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
 

You're gonna need to spell it out for me, please. Pictures of other guy's scan data never makes sense. 1. If the exhaust is cracked and you plug it what happens? 2. if the exhaust is not cracked and you plug it what happens?

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Albin Diagnostician
Leavenworth, Washington
Albin Default
 

That crack is letting a little atmosphere into the front of the cat. This extra oxygen is seen by the oxygen sensors,,, or AFR sensors and skews the trims. Sticking the rag in the tail pipe creates enough pressure to stop the exhaust from sucking the air in, which lets the system work as if there is no exhaust leak. By having the exhaust analyzer in the pipe, you can verify the oxygen sensors…

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
 

So if the O2 goes rich, when you plug the exhaust, then there is a leak. IF there is NO leak would, would the O2 stay lean? If the 02 was broken, it would stay lean either way, (leak or no leak) and nobody has an exhaust analyzer out here, so I'm unsure if the test would work for me, unless it goes rich, then I know there is a leak. I've never tried smoke for an exhaust leak, but I was…

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Jamey Owner/Technician
Ventura, California
Jamey Default
 

Geoff, you can also take an air blower tip (metal preferably) and blow high pressure air all around the welds and joints. If there is a leak, you will see Stft react.

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
 

Air? I was gonna pour gasoline on it.....bwahahahaha

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Aaron Diagnostician
Council Bluffs, Iowa
Aaron Default
 

I have had good luck with smoke in the exhaust, I have been able to find small "pinholes" with it. Just need to let it run for a while to get the air pushed out, so it is not always a real quick test if it is the only thing you are doing, but if you have enough to do while it runs it works good. I like it because you can check it cold and don't have to mess with hot exhaust, but so would your…

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Gary Technical Support Specialist
Barrington , New Jersey
Gary Default
 

Geoff the O2 sensor 2 would not be lean in the first place. If the the cat was working and no crack. So there would be no change rag on or off

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
 

What if the O2 is stuck? (broken)

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Hollis Technician
Boulder, Colorado
Hollis Default
 

Stuck / Broken as in Biased or Lazy ? Wait a minute, I'm thinking of something else, confusing times when I start anthropomorphizing fuel control issues :O)

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Randy Analyst
Denver, Colorado
Randy Default
   

I feel your pain and agree with you about scan pictures but it’s the best I can do. Albin answered but to make sure you understand, I plugged the tailpipe not the crack in the exhaust so from there, answer to 1 is if leak is affecting sensor then it will go rich. Answer to 2 is nothing will change as far as o2 voltage or lambda.

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Geoff Diagnostician
Lahaina, Hawaii
Geoff Default
 

OK gotcha Randy, thank you. I was thinking back-pressure would make it rich, even if there was no leak, BUT it wouldn't be lean to begin with, with no leak, unless O2 is broken. I suppose the standard scenario is "guy already replaced four O2 at last shop" and now it's our turn. Of course he's outta money now....sorry off topic! - lol

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Albin Diagnostician
Leavenworth, Washington
Albin Default
 

Randy, It has been several years since I was able to take a trip to Denver and get a headache sitting in one of your classes. Thanks for the opportunity that YOU gave me to go to the next level!!!.

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Randy Analyst
Denver, Colorado
Randy Default
 

You are welcome, that was 11 years ago, the last real class I did. I don’t teach classes anymore but those were some fun times.

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Jamey Owner/Technician
Ventura, California
Jamey Default
 

Nice job Randy. thanks for the detailed screen shots. On another note, I had 2, 2010/11 Jeeps last week that would not set the readiness monitor for the O2. Neither were willing to spend money on diagnostics at the time. I’ll give em about 1000 miles driving and they will be back!

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Randy Analyst
Denver, Colorado
Randy Default
 

Did you try my trick? I have completed a lot of those since figuring it out.

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Jamey Owner/Technician
Ventura, California
Jamey Default
 

No, but I will when they drop one off. They just have to accept the fact that testing cost money. They are not my regular customer..... yet!

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Hollis Technician
Boulder, Colorado
Hollis Default
   

Ah ha ! I like. Nailing down these Post Cat F.T. DTC's has been on your Bucket List for quite sometime....:O)

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Gary Technical Support Specialist
Barrington , New Jersey
Gary Default
 

Used the same test on a Hyundai with a cracked flex pipe that was making no noise. Came in with a P0139 (again) B2S2 showed low voltage until I raged the tailpipe then shot up to .8 until the rag was removed.

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