SOLVED! Excessive STFT Issue

Jkb242

Original poster
Member
May 19, 2019
239
CLT
Don’t recall ever seeing this previously on my 2006 Denali but live data yesterday, reveals 99.2% at S2 both banks. Have had P0430 pointing to CAT on bank 2. Long and sort term trims otherwise look normal. O2 downstream sensor not reporting anything close to a steady reading but varying between both extremes, monitoring live data. Before jump to CAT replacement, I was looking for any suggested testing while monitoring upstream and downstream sensors, to isolate other issues that could be at fault or at least contributors.

Performance and MPG is completely normal across all loading ranges. Coolant temp sensor is reporting 60⁰ at cold startup with ambient temp around 40⁰. Replaced sensor about two weeks ago because of other unrelated issues.

Much thanks in advance for input.
 

TollKeeper

Supporting Donor
Member
Dec 3, 2011
8,047
Brighton, CO
Well that was quick, what did it end up being?
 

Jkb242

Original poster
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May 19, 2019
239
CLT
No interest apparently which is really surprising given the tremendous responses in the past. I think I should have more brief. I think a repost in a much shorter format might be great. Did I marked this correctly as solved? Hope I’m not violating any rules reposting in brevity.
 

Jkb242

Original poster
Member
May 19, 2019
239
CLT
REVISION OF PREVIOUS POST MARKED “SOLVED”.

The intent here is simply to describe the help I am seeking in a more concise manner. The issue is excessively large reported short term fuel trim on both downstream bank sensors. Reporting 99.2%! Engine performance is very good, MPG normal, no noticeable drive issues or symptoms of any kind, no codes except Cat Eff bank 2, bank 1 no code. Appreciate any assistance in identifying actual issue.
Thanks!!
 

Blckshdw

Moderator
Nov 20, 2011
10,665
Tampa Bay Area, FL
:confused: So you didn't solve the problem? Reposting a new thread due to lack of desired responses in another thread, is something we want to avoid. I'm going to merge your other post to this thread to keep responses in 1 place and adjust your thread title, that may have thrown people off.
 

TollKeeper

Supporting Donor
Member
Dec 3, 2011
8,047
Brighton, CO
My understandings of the datalog, it should not even be able to report more than 26.8, and has already set a CEL at 16.2 in closed loop. The fact that its reporting 99.2 is mind boggling. I would start with checking OHM resistance at the O2 sensor itself. It may have just reached the end of its life expectancy? I will say that I am not very well versed in the PID's world, and the learning curve is insane. But I am learning. To me, either the sensor has gone wonkey, OR, someone removed the cat, and put a straight pipe in.
 

Jkb242

Original poster
Member
May 19, 2019
239
CLT
:confused: So you didn't solve the problem? Reposting a new thread due to lack of desired responses in another thread, is something we want to avoid. I'm going to merge your other post to this thread to keep responses in 1 place and adjust your thread title, that may have thrown people off.

No, same issue but thanks for offering to merge, possibility my idea to add clarity wasn’t the best choice but re-reading my post led me to conclude others may not have thought my request wasn’t that clear.
 

Jkb242

Original poster
Member
May 19, 2019
239
CLT
Purchased this car used. The exhaust system was replaced with I understand is a low back pressure muffler, which makes it sound a little louder than stock but pretty sure both CATS are there. I have owned it nearly two years and never had the P0430 code until recently.
The 5.3L has the DOD and associated oil issues, which nearly everyone who had kept theirs beyond 120K miles has had as well. Have not replaced the valve cover with the external PVC (many reported this as a fix) so there is some oil being burned as a result of the DOD. Possibly, this could have an effect on the downstream O² or the CAT. This is why I am seeking some positive diagnostic assistance in determining the exact cause. Keep in mind the SHTRFT1 and SHTRFT2 are both normal as are the LTRFT1 & LTRFT2.
 

Jkb242

Original poster
Member
May 19, 2019
239
CLT
REVISION OF PREVIOUS POST MARKED “SOLVED”.

The intent here is simply to describe the help I am seeking in a more concise manner. The issue is excessively large reported short term fuel trim on both downstream bank sensors. Reporting 99.2%! Engine performance is very good, MPG normal, no noticeable drive issues or symptoms of any kind, no codes except Cat Eff bank 2, bank 1 no code. Appreciate any assistance in identifying actual issue.
Thanks!!

UPDATE--NEW INFORMATION DISCOVERED 1/24/2020

1) At idle ~600 RPM, LTFT1 & 2 both report -8.0% to over -10.0% and are fluctuating like SHTFT would normally behave while SHTFT1 & 2 report well within normal range both positive and below 3.0%.

2) At anything over ~800 RPM, LTFT1 & 2 report positive fuel trims in the normal range below 5.0%. Performance seems normal in all other aspects.

It would seem and idle circuit issue, but idle feels normal, not rough and no codes reported except the infamous P0430 B2S2 (Cat Efficiency) which I suspected may be due to the O2 sensor and possibly has now failed completely. Just not sure. Maybe this new issue should be considered a separate issue but I believe it to be related to the original concern of the P0430 code when all other fuel trims looked normal. Thanks..
 

Blckshdw

Moderator
Nov 20, 2011
10,665
Tampa Bay Area, FL
So are you going to try testing the O2 sensor like previously suggested? Or change them outright?
 

Jkb242

Original poster
Member
May 19, 2019
239
CLT
Indeed I now recall seeing that suggestion and certainly is a good suggestion. I became distracted from that response given the shockingly abnormal discovery yesterday.

I am not certain if the 02 downstream sensors are normal or wideband which as I understand behave opposite to lean and rich detection. Can you tell me if both downstream sensors are normal or wide band? Also since both banks reported abnormal fuel trim behavior in the new observation yesterday, both downstream sensors failing seems a bit unlikely. However, possible I was wondering with this new information is there a more likely broader cause that would cause both banks to behave abnormal.

I may have to have them replaced since I don't think they are reachable for any testing without a lift and do not have access to one. Can you tell me if simply raising one side of the car then the other will allow the access I need?
 

Maverick6587

Member
Dec 16, 2018
730
Sterling Heights, Michigan
How many miles do you have on it and do you know if the CAT has ever been replaced? Have you cleaned the throttle body lately (and unplugged the battery for at least 30 minutes)?

Honestly for the price of the O2 sensors, I'd replace all of them before I threw over $300 into just the CAT. Looks like you can get all 4 of them for around $90 and that's the amazon price for Bosch.

I would start with a vacuum leak testing first though. As well as cleaning the MAF and MAP, with MAF cleaning spray only, CRC makes a decent Mass Air Flow cleaner. Any other cleaner could harm the sensors.

In the following video he shows you two different methods to do vacuum testing. I can't remember if he also shows or explains that you can use soap and water, but that's another method. I'm finishing a report at work and didn't have time to re-watch the video.

 
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Jkb242

Original poster
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May 19, 2019
239
CLT
Thanks, really appreciate the input. I will follow your suggestion as I suspect one or both downstream sensors. I believe I have two cats since this is a 5.3 V8. I was just reading an article about testing the MAP sensor since it and MAP control both banks and should be suspects as you point out. Thanks for taking the time to respond, from your busy day.

The currently mileage is around 147K, I think but had the carfax report when I purchased to see what recorded service had been reported before I purchased. I have since lost that but hoping to recover it but remember seeing the CAT and O2's were replace along with plugs all around the 100K mark.
 

Maverick6587

Member
Dec 16, 2018
730
Sterling Heights, Michigan
If both CATs and O2's have been replaced I would definitely start with cleaning the MAF and MAP! Unless they were replaced with non-ACDelco/Bosch/Delphi O2's sensors. The Vortec is VERY sensitive to non-OEM parts.
 

TollKeeper

Supporting Donor
Member
Dec 3, 2011
8,047
Brighton, CO
He is correct.. Only go with ACDelco, or NTK/NGK (supplier for GM), or Delphi.

Start with cleaning the MAF and MAP.
 

Jkb242

Original poster
Member
May 19, 2019
239
CLT
UPDATE--NEW INFORMATION DISCOVERED 1/24/2020

1) At idle ~600 RPM, LTFT1 & 2 both report -8.0% to over -10.0% and are fluctuating like SHTFT would normally behave while SHTFT1 & 2 report well within normal range both positive and below 3.0%.

2) At anything over ~800 RPM, LTFT1 & 2 report positive fuel trims in the normal range below 5.0%. Performance seems normal in all other aspects.

It would seem and idle circuit issue, but idle feels normal, not rough and no codes reported except the infamous P0430 B2S2 (Cat Efficiency) which I suspected may be due to the O2 sensor and possibly has now failed completely. Just not sure. Maybe this new issue should be considered a separate issue but I believe it to be related to the original concern of the P0430 code when all other fuel trims looked normal. Thanks..

The anomaly reported last week regarding the LONGTRFT & SHTRTRFT at idle and the sharp reversal these fuel trims above 800RPM was corrected based on initial test this AM. No drive test yet but confident that will confirm resolution as well. Trouble was a vacuum leak at what I believe is the MAP sensor and the valve cover vent on bank 1.

I had repaired the broken “formed plastic” vacuum tubing by removing it and replacing it with a piece of neoprene 3/8” hose. The hose had worked loose and could be easily removed at the MAP sensor especially with the oil residue (originating from the infamous DOD and lack of a PCV in the bank 1 valve cover) that coated the inside of the hose fitting. I also installed small stainless steel hose clamps at the hose functions insuring a tight fit.
The other issue regarding the SHTRTFT B1 & B2 at sensor 2 is still at 99.2%. I suspect after a test drive today, I will also see TC P0430 again.
Because both banks at S2 show identical readings I suppose both O2 sensors could be bad but it just seems odd. Confirmed 12v supply feeds to MAP and MAS sensors and to both O2 sensors is alive. Thanks to all who are continuing to monitor and assist.
 

budwich

Member
Jun 16, 2013
2,027
kanata
not sure I followed all of the "upstream / downstream" measurements / comments but it is good that you may have found issue with the MAP sensor (or area there of... ie vacuum leak). From my "read", the MAP (and its readings) have impact during idle while the MAF, not so much / none. At "speed", the system "blends" both to work something out.... of course, more complex than those simple statements.

Based on my limited 4.2 monitoring, the upstream values of voltages and trims are useful and should respond according (throttle) plus "switching" (oscillation). However, the downstream should not be so "responsive" but drift / "plateau" thru levels if things are working in the cat / post cat (o2 sensor itself). I am not sure how the trim numbers for downstream actually relate since it is my understanding that the measurements are not used to control any fueling "adjustments"/ indicators (ie. calculated numbers) as opposed to "raw voltages". I am not even sure that trim values for downstream sensors are even valid or are just a "figment of the scanner / reader". IF the voltages of the downstream are oscillating "in step" with the upstream, to me that would indicate "dead cats"... especially at idle when the system should be able to handle the exhaust flows relatively well.

sorry if I misinterpret / misread what you have stated / asked... disregard if so.
 
Last edited:

Jkb242

Original poster
Member
May 19, 2019
239
CLT
I think you are indeed correct. I believe the downstream monitoring should show an average voltage somewhere around 0.45 and mostly constant. I believe mine are varying more than they should. I have not looked at the B2 sensor voltage yet as I was watching the fuel trims mainly this AM. The voltage readings at B2 are part of the diagnosis I realize. I am a bit limited to playback of live data after a drive not feeling safe monitoring the scanner while driving.
I have the PDF’ provided so graciously by Mooseman which is a tremendous help but the illustration showing the MAP sensor does not agree with the location I see on my 5.3L.

ill shoot a picture of this and post here for confirmation. The cats were replaced in mid 2015, with likely some knock-off but I need to Get the car up and do an inspection. I did hear rattling a few months back like a loose muffler clamp but it might have been the cat’s dying breath too. I believe on the 5.3L the two cats “T” together at the output into one tail piece leading to the muffler. It this is true I think both cats were replaced but unsure about sensors, they could be originals!
 

Blckshdw

Moderator
Nov 20, 2011
10,665
Tampa Bay Area, FL
Forgive me if I missed where you stated this part, but how are you monitoring your sensor data? On your phone via app, or a standalone scanner device? Many of us use Torque on our phones or tablets, I ran into some issues last summer and was able to use a screen recording app on my Android HU to record the display so I could drive without being distracted. Really came in handy. :twocents:
 

Jkb242

Original poster
Member
May 19, 2019
239
CLT
Forgive me if I missed where you stated this part, but how are you monitoring your sensor data? On your phone via app, or a standalone scanner device? Many of us use Torque on our phones or tablets, I ran into some issues last summer and was able to use a screen recording app on my Android HU to record the display so I could drive without being distracted. Really came in handy. :twocents:
I use a scanner that offers some command testing but not like a full >$1800 device like SnapOn or other professional models. Mine is made by Foxwell it’s a 630 Pro that offers extended testing for Seatbelt and ABS testing in addition all of the standard low end scanners.
 

Blckshdw

Moderator
Nov 20, 2011
10,665
Tampa Bay Area, FL
I guess, depending on what odds and ends you have laying around, you could try propping up your phone in a position to record what's displayed on the scanner, so you can review the live data after a test drive.
 

Jkb242

Original poster
Member
May 19, 2019
239
CLT
Forgive me if I missed where you stated this part, but how are you monitoring your sensor data? On your phone via app, or a standalone scanner device? Many of us use Torque on our phones or tablets, I ran into some issues last summer and was able to use a screen recording app on my Android HU to record the display so I could drive without being distracted. Really came in handy. :twocents:
Not a bad idea thanks. My scanner records the live data internally allowing a playback in the house with a standard mini USB cable attached to any phone charger. Pictures of the frame data at any time can be taken for sharing but I haven’t found this particularly useful since any parameter generated by the ECM can be viewed. Thanks
 

budwich

Member
Jun 16, 2013
2,027
kanata
Depending on the amount of money and time you want to spend ... plus what you have hanging around.... people use their phone / tablets with android get torque app with a bluetooth OBD interface to monitor things which can readilly provide "real time graphing" / viewing of o2 voltages (all 4 at once). This can be done for less than $20 assuming a compute device is handy.

Further, depending on costs / availability, you can "shoot" the cats with a thermal / IR temperature reading device. Some small hand held temperature devices can be had for $10-20. Basically, read the "cold" temperature and then do a freeway run and then read the "hot" temperature. This may tell you if the cats are "running" or if they have "meowed". Go from there.
 

Jkb242

Original poster
Member
May 19, 2019
239
CLT
Depending on the amount of money and time you want to spend ... plus what you have hanging around.... people use their phone / tablets with android get torque app with a bluetooth OBD interface to monitor things which can readilly provide "real time graphing" / viewing of o2 voltages (all 4 at once). This can be done for less than $20 assuming a compute device is handy.

Further, depending on costs / availability, you can "shoot" the cats with a thermal / IR temperature reading device. Some small hand held temperature devices can be had for $10-20. Basically, read the "cold" temperature and then do a freeway run and then read the "hot" temperature. This may tell you if the cats are "running" or if they have "meowed". Go from there.
Yes that’s definitely an input in diagnosing root cause. I have a thermal scanner but I have seen fairly wild deviations in the readings I’ve taken in the past, not on the CAT but other things. For this reason, I don’t feel this particular scanner would be that useful but I believe it could show the Δ between upstream and downstream temps for a reasonable indication of normal behavior. Thanks!
 

budwich

Member
Jun 16, 2013
2,027
kanata
:-( not sure what type you have.... I have both an imager and a couple of "eye" units (much like a small ir remote control). All have been good. Depending on the unit, "reflectivity" does play a role but in general measurements, it shouldn't matter and at the level that is involved here, very coarse measurements should work fine. Also since you have two cats, comparisons as a "wet finger" check should also help "certify" measurements are OK. Still further, if a question exists, crawling under another vehicle to confirm things would work too. :smile:
 

Jkb242

Original poster
Member
May 19, 2019
239
CLT
:-( not sure what type you have.... I have both an imager and a couple of "eye" units (much like a small ir remote control). All have been good. Depending on the unit, "reflectivity" does play a role but in general measurements, it shouldn't matter and at the level that is involved here, very coarse measurements should work fine. Also since you have two cats, comparisons as a "wet finger" check should also help "certify" measurements are OK. Still further, if a question exists, crawling under another vehicle to confirm things would work too. :smile:
Indeed thanks!
Don’t recall ever seeing this previously on my 2006 Denali but live data yesterday, reveals 99.2% at S2 both banks. Have had P0430 pointing to CAT on bank 2. Long and sort term trims otherwise look normal. O2 downstream sensor not reporting anything close to a steady reading but varying between both extremes, monitoring live data. Before jump to CAT replacement, I was looking for any suggested testing while monitoring upstream and downstream sensors, to isolate other issues that could be at fault or at least contributors.

Performance and MPG is completely normal across all loading ranges. Coolant temp sensor is reporting 60⁰ at cold startup with ambient temp around 40⁰. Replaced sensor about two weeks ago because of other unrelated issues.

Much thanks in advance for input.
Would Appreciate Comments on Pictures:
1) Engine Bay-any comments regarding missing pieces, no engine cover missing
2)Blue arrow-believe this the MAP sensor
3) Blue arrow-vacuum port to valve covers where oil residue, was found causing vacuum hose to loosen, Red arrow- shows neoprene vacuum line added leading to valve covers
4) Blue arrow- bracket that seems to have missing hardware previously attached or was a tie point for harness routing.

I have tried to identify theses areas myself from engine bay diagrams but there isn’t sufficient detail in the drawings to positively address the correctness for which I’m seeking clarification.
Thanks!!

DD9C68E1-2A54-4195-9407-887638AFBB2A.jpeg

FAD94BA4-64C2-462D-AF00-67E0D7916FB6.jpeg
66B15F47-3B74-4299-B78F-63B24CD8DF07.jpeg
BEDB059D-78E4-4FAF-AD32-2F796995FF87.jpeg
 

Jkb242

Original poster
Member
May 19, 2019
239
CLT
Forgive me if I missed where you stated this part, but how are you monitoring your sensor data? On your phone via app, or a standalone scanner device? Many of us use Torque on our phones or tablets, I ran into some issues last summer and was able to use a screen recording app on my Android HU to record the display so I could drive without being distracted. Really came in handy. :twocents:
Bickshdw, I don’t have any android based tablets required for the TORQUE app. It like it however. Are you aware of a comparable IOS app like “Engine” that provides the same in-depth of analysis as TORQUE? The pain of being tethered to the cable connecting my scanner is becoming annoying and wireless looks awfully tempting for just monitoring.
 

Mooseman

Moderator
Dec 4, 2011
25,262
Ottawa, ON
We have a whole section dedicated to scanners and OBD.


OBD fusion seems to be the only game in town for IOS using a wifi ELM327 adapter.


edit: There is also Dash Command:

 
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Jkb242

Original poster
Member
May 19, 2019
239
CLT
We have a whole section dedicated to scanners and OBD.


OBD fusion seems to be the only game in town for IOS using a wifi ELM327 adapter.


edit: There is also Dash Command:

Thank you, I apparently missed that section. The Torque Pro app is available for from the Apple App Store BUT it received terrible reviews most all were the same no connection. Going to read up on the material here before purchase.
 

Jkb242

Original poster
Member
May 19, 2019
239
CLT
How many miles do you have on it and do you know if the CAT has ever been replaced? Have you cleaned the throttle body lately (and unplugged the battery for at least 30 minutes)?

Honestly for the price of the O2 sensors, I'd replace all of them before I threw over $300 into just the CAT. Looks like you can get all 4 of them for around $90 and that's the amazon price for Bosch.

I would start with a vacuum leak testing first though. As well as cleaning the MAF and MAP, with MAF cleaning spray only, CRC makes a decent Mass Air Flow cleaner. Any other cleaner could harm the sensors.

In the following video he shows you two different methods to do vacuum testing. I can't remember if he also shows or explains that you can use soap and water, but that's another method. I'm finishing a report at work and didn't have time to re-watch the video.

*********UPDATE*******

I have not been able to do further testing for a number of reasons but have continued to monitor the data real time. I purchased a Delphi “guaranteed to fit” part from amazon and today discovered that the connector on the new Delphi is square while the existing harness connector is triangular. i also noticed several parts that were “direct replacements” also contained a square connector while other “off brand” sensors had the correct triangular fitting. How can this be?? I check Rock Auto and they too showed the BANK 2 sensor 2 had a square fitting? Can you help me shed some light on this issue? The harness on this sensor appears to be original with no indications of a splice
That would hint someone swapped the harness for something other than stock. Thanks in advance!!
 

Jkb242

Original poster
Member
May 19, 2019
239
CLT
*********UPDATE*******

I have not been able to do further testing for a number of reasons but have continued to monitor the data real time. I purchased a Delphi “guaranteed to fit” part from amazon and today discovered that the connector on the new Delphi is square while the existing harness connector is triangular. i also noticed several parts that were “direct replacements” also contained a square connector while other “off brand” sensors had the correct triangular fitting. How can this be?? I check Rock Auto and they too showed the BANK 2 sensor 2 had a square fitting? Can you help me shed some light on this issue? The harness on this sensor appears to be original with no indications of a splice
That would hint someone swapped the harness for something other than stock. Thanks in advance!!

ANOTHER UPDATE REGARDING O2 CONNECTOR

After a bit more looking at other sites like Autoxone and Advance Auto Parts, the triangular and square connector issue seems to boil down to”design 1” or “design 2” Amazon’s search engine seems to default to design 1 for my Denali application. design 2 is a triangular shaped connector while design 1 is square in almost every search I did.
Searching Amazon by your car year, model and engine won’t produce reliable results but searching by part number for O2 sensors is best. But unless there is a number on the existing part AND assuming you removed it already, there isn’t any other way to know which design (1 or 2) was used in a particular application.
Unless there is something I’m missing here, this might be good info to pass along to other members in a sticky regarding O2 sensors.

In my particular case I was unable to remove the sensor before purchase. Only when my shop, who was going to swap the old one for the new one, remove the connector, which I was unable to reach, did I discover I had a design 2 application.
 

Jkb242

Original poster
Member
May 19, 2019
239
CLT
:-( not sure what type you have.... I have both an imager and a couple of "eye" units (much like a small ir remote control). All have been good. Depending on the unit, "reflectivity" does play a role but in general measurements, it shouldn't matter and at the level that is involved here, very coarse measurements should work fine. Also since you have two cats, comparisons as a "wet finger" check should also help "certify" measurements are OK. Still further, if a question exists, crawling under another vehicle to confirm things would work too. :smile:
Thanks to all who responded to previously reported issues, now all of these pretty much resolved as briefly summarized below:

Fuel trim issues:
Resolved by carefully comparing both bank1 and 2 and viewing live data with two separate scan tools. Found small vacuum leak at Evap sensor mounted at center of engine, added SS clamp to secure neoprene hose and cleaning MAS.

CAT bank 2:
Replaced Downstream O2 Sensor with AC Delco OEM part. Original sensor was constantly producing wildly fluctuating voltages like a faulty CAT from 0.1 to 0.9, no normal range reading at any load or at any normal RPM. Now reports normal readings at all engine loads and RPM. Between engine loads and RPM readings are outside of normal but quickly return to quiescent normal state once new load and ROM is reached.

Front End Repairs:
Found a very reliable trustworthy mechanic to replace all ball joints and control arms and bushings, $1250.00. Rides and handles quite nicely.
That seemed like a fair price. Have rear air inflatable original shocks did not replace but feel they offer a more ride with advantage of load leveling. Did not replace front struts which show no indication of issues.

Thanks to all again for input.
 

aslusser

Member
Jul 25, 2023
1
Eaton, ohio
Thanks to all who responded to previously reported issues, now all of these pretty much resolved as briefly summarized below:

Fuel trim issues:
Resolved by carefully comparing both bank1 and 2 and viewing live data with two separate scan tools. Found small vacuum leak at Evap sensor mounted at center of engine, added SS clamp to secure neoprene hose and cleaning MAS.

CAT bank 2:
Replaced Downstream O2 Sensor with AC Delco OEM part. Original sensor was constantly producing wildly fluctuating voltages like a faulty CAT from 0.1 to 0.9, no normal range reading at any load or at any normal RPM. Now reports normal readings at all engine loads and RPM. Between engine loads and RPM readings are outside of normal but quickly return to quiescent normal state once new load and ROM is reached.

Front End Repairs:
Found a very reliable trustworthy mechanic to replace all ball joints and control arms and bushings, $1250.00. Rides and handles quite nicely.
That seemed like a fair price. Have rear air inflatable original shocks did not replace but feel they offer a more ride with advantage of load leveling. Did not replace front struts which show no indication of issues.

Thanks to all again for input.
What % number did you get after replacing O2 sensor?
 

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