Fuel trims and hesitation after new cats?

Tiny
BILL MARSHALL
  • MEMBER
  • 2011 INFINITI G37
  • 3.7L
  • V6
  • 2WD
  • MANUAL
  • 92,000 MILES
My car developed Nissan's infamous cat rattle about 2 months ago (the substrate breaks into pieces and sounds like rocks in a tin can) and I noticed both flex pipes had holes worn through them as I was under it. Replaced the cats with Berk high flow cats (50 state compliant, built in anti-foulers, guaranteed to pass emissions, which it did) and also added a full cat back exhaust. Excessive vibration from using an impact on the rusted "demon bolts" from cat to manifold damaged B2S1 O2 so, I replaced it with a Denso 234-9135.

About a week ago, I get a P2A03 code, B2S1 range/performance. The first thing the FSM says to do is clear the fuel mixture learned values so, I did. The code hasn't come back, but now I get a hesitation when I accelerate from a stop and when I step on it from light throttle. I was checking the live data while driving tonight and both upstream AFR sensors fluctuated from about 1.5V to 3V, STFT B1 and B2 both stayed within +-5, LTFT B1 was about 1.5 and LTFT B2 was 9.3. When I turned off the engine and ignition, all trims showed a value of 0 except LTFT B2, which held a 3.9.

Isn't short term based off of the upstream readings and long term based off of the downstream readings? Is it normal for the ECU to hold a trim value when the ignition is off and would this be causing my hesitation? Thanks.
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Sunday, November 9th, 2025 AT 9:56 PM

14 Replies

Tiny
KEN L
  • MASTER CERTIFIED MECHANIC
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So the hesitation seems like it would be from the system trying to lean the fuel mixture out from the rich reading of 9.3, (both STFT and LTFT are based almost entirely on the upstream (Sensor 1) AFR sensors.)

Code P2A03 - O2 Sensor Circuit Range/Performance (Bank 2, Sensor 1)

So this will be a sensor issue sensor 1 bank 2 or you have a leaking/bad fuel injector, I would remove the spark plugs to see which one is dark or black in color, this will be the cylinder having the issue, also since the spark plug is out I would do a compression test on the cylinder as well. Please go over these guides to help as well

https://www.2carpros.com/articles/why-an-automotive-engine-will-run-rich

and

https://www.2carpros.com/articles/how-to-test-engine-compression

Please upload a short video in your response of the problem, so I can see what's going on :)
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Tuesday, November 11th, 2025 AT 10:27 AM
Tiny
BILL MARSHALL
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Ken,

Thank you for the prompt reply. I have not had the time to pull the plugs yet, however, I was watching some more live data and now the STFT for B1 and B2 hover around +5 at idle, fluctuating from anywhere between -10 and +15. LTFT B1 is now +4.X and B2 is +11.X. A/F Alpha B1/B2 ranges from 97% - 120% and both downstream sensors are a constant 0.28V (HO2S2 MNTR B1/B2 = LEAN with no change). The car runs fine, other than the aforementioned hesitation and I don't hear an exhaust leak, plus, there are no DTCs present. I'll check for vacuum leaks in the intake system this weekend.

So that we're on the same page, the car is my daily but doesn't get driven very far as I work from home. I've had it 6 years and have put 20k mi on it. It has Stillen long tube cold air intakes that put the filters in front to the radiator, Concept Z Performance PCV delete kit, throttle bodies from a 2017 QX50, deleted throttle body coolant lines, new upper plenum gasket, valve cover gaskets, spark plugs, spark plug tube seals and, as mentioned in my original post, B2S1 A/F sensor, high flow cats and cat-back. All the work has been done by me within the last 10k mi.
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Saturday, November 15th, 2025 AT 5:22 AM
Tiny
KEN L
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That fact that bank 2 is at +11 is tell us you have one or more fuel injectors and can be leaking or there is an exhaust leak on bank 2 which will fool the computer into thinking the bank is lean and it will richen up the fuel mixture. I would remove the spark plugs on bank two to see if one or all are dark compared to bank 1. If you find one that is dark you found the problem cylinder. Please do the compression test as well as long as the spark plugs or removed. This is so we don't chase our tail when the engine is having a problem, (low compression). Let me know what you find.
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Monday, November 17th, 2025 AT 8:38 AM
Tiny
BILL MARSHALL
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  • 38 POSTS
I removed the plugs on bank 2, but they look fine to me (see images). I tried doing a compression test, but the kit I have is the thread type and it doesn't fit down into the spark plug tube. Apparently, I need some sort of adapter rod with a rubber tip that goes into the tube. I don't see how that would hold the pressure if it doesn't screw in. I forgot to mention that I've also replaced the fuel damper assembly a while back due to it leaking (this car actually uses 3 of them. 1 before the rail and 1 on each bank's rail directly above the injectors. I replaced the one before the rail.)
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Tuesday, November 18th, 2025 AT 9:57 AM
Tiny
BILL MARSHALL
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I don't know why it's not loading the images. It shows them before I hit the reply button.
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Tuesday, November 18th, 2025 AT 9:58 AM
Tiny
BILL MARSHALL
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I downsized them, we'll see if that makes s difference.
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Tuesday, November 18th, 2025 AT 10:01 AM
Tiny
KEN L
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Yes, it is a display issue, they are working on it and they will show up in the next few days. The videos still work if you want to do a short video and upload it here as well.
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Tuesday, November 18th, 2025 AT 10:46 AM
Tiny
BILL MARSHALL
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I just made a short video clip of the images of the plugs.
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Tuesday, November 18th, 2025 AT 1:05 PM
Tiny
KEN L
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Nice, thank you for the video, so the last spark plug looks lean, no or little fuel this will be he cylinder having the issue. This is why the system is trying to richen the mixture, if the compression is good I would replace the fuel injector on that cylinder. Clear the codes and see what happens.
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Wednesday, November 19th, 2025 AT 8:26 AM
Tiny
BILL MARSHALL
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Is that because the tip isn't brown like the other 2? Do you know what adapter I need to do the compression test?
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Wednesday, November 19th, 2025 AT 10:52 AM
Tiny
KEN L
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This video will show you how and what adapter you need.

https://www.youtube.com/watch?v=KOxIm1F8AW0

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Thursday, November 20th, 2025 AT 9:07 AM
Tiny
BILL MARSHALL
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I have a similar kit as in the video, but the FSM says I need one of these
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Thursday, November 20th, 2025 AT 4:35 PM
Tiny
BILL MARSHALL
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I figured it out. Compression numbers on C2, C4 and C6 were all between 240 - 245 psi. The last plug in the video was C6. I should just replace the injector then? The book says the only way to test the injectors, other than listening for an energized click, is to pull them out, put them in a container, still connected to the rail, and fire them to make sure they all flow the same.

Btw, LTFT B2 is now +16.
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Thursday, November 20th, 2025 AT 7:43 PM
Tiny
KEN L
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Yep, that is a good way to compare the spray pattern, but it looks like from this LTFT B2 reading the injector is clogged, here is a guide that can help you test the injector as well.

https://www.2carpros.com/articles/how-to-test-a-fuel-injector

Please go over this guide and get back to us.
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Friday, November 21st, 2025 AT 9:05 AM

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