2008 Buick Lucerne P0420 Problem

William Diagnostician Virginia Posted   Latest   Edited  
Resolved
Driveability
2008 Buick Lucerne CXL 3.8L (2 L26) 4-spd (4T65-E) — 1G4HD57218U157905
P0420 — Catalyst System Efficiency Below Threshold Bank 1
Sets P0420 Within 1 Day

This my personal vehicle. About 9 months ago the OEM cat started setting P0420. After testing it was replaced with aftermarket cat. This month it set P0420 again. After testing the cat was replaced under warranty. The new cat set P0420 right away. I have repeated all the tests done before with the same result: Bad cat. I am posting this in hopes that someone may see something I missed. Both O2's were replaced with first cat (age) and still show good. All monitors have run with no other failures.

Attached are some Snap-On scanner files and snapshots, as well as a data shot from another 3.8 with similar O2 patterns taken when the cat was bad, supposedly running the cat test.

Thanks!!

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Rusty Owner/Technician
Massachusetts
Rusty Default
 

If the fuel trims are normal, I’d put an OEM cat on it. The PCM doesn’t like those apparently In my experience, a healthy cat at steady-state causes the S2 to stay very stable around 700mV

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William Diagnostician
Virginia
William Default
 

Yes fuel trims re +/- 3% both long & short trim. we have used aftermarket cats on many older US vehicles with little problems. Not the case with mine.

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Jamie Owner
South Carolina
Jamie Default
 

We had a 2015 Chevy Cruze 1.4 that did the same thing. We replaced 2 aftermarket converters before replacing with the OEM converter. The OEM converter solved the problem.

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John Diagnostician
Texas
John Default
   

Catalysts fail in different ways. by Looking at the data it appears that the switching back and forth of the rear o2 is a clear indicator of the cats inefficiency. OEM cats are the only way to go IMO. do You happen to have freeze frame data? also, 9 months with the first replacement prior to code setting, Is there any concern over something killing the cat?

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William Diagnostician
Virginia
William Default
 

No issues were found. Not losing coolant and used 1 qt of oil in 5-6000 miles. The fact that the second replacement failed the test 2 days after installing would suggest no issues with damage. And proper breakin procedures were performed. i have the freeze frame data. Nothing was out of wack with it. I’ll try to post it tomorrow. Thanks!!

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Lewis Technician
California
Lewis Default
 

William , please explain how you “break in a cat” ?

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William Diagnostician
Virginia
William Default
 

It is a procedure from many aftermarket cats. Basically you start and let it warm for 5 minutes, then rev to 2000 for 5 minutes, then idle another 5. It expands the packing surrounding the substrate is what I read.

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Michael Owner/Technician
Texas
Michael Default
 

I know GM had issues a TSB that mentioned a 200 mile burn in or something like (Degas) for Cat's that was programmed into new vehicles, that would not test a Cat until then.

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Kenneth Owner/Technician
Arkansas
Kenneth Default
 

f01​.​justanswer​.​com/jp_greenville1…​.​pdf

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William Diagnostician
Virginia
William Default
 

I’ll check this, but it‘ s a car, not a truck. Also none of the cats were lose or damaged. I used a bore scope to check them. The faces were fine and clean.

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Nathaniel Diagnostician
Ontario
Nathaniel Default
 

Not saying whether it relates to your situation or not, but it states that it applies to just about every GM gasser ever: Models: 2016 and Prior GM Cars and Gasoline-Powered MST Light Duty and HD Trucks

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Lewis Technician
California
Lewis Default
 

but it states that it applies to just about every GM gasser ever totally ridiculous .

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Paul Mechanic
Illinois
Paul Default
 

Is this a California certified emissions vehicle or federal? If California, you'll need OEM to satisfy PCM. Check mode 6 and see how bad it's failing and of course no exhaust leaks. The rear O2S looks a bit lazy. If you don't know how, look up dtc 420 and you'll read how to run cat monitor in bay and you can watch what the O2s do during test-there will be no missing when test is running. I know…

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William Diagnostician
Virginia
William Default
 

It is a Federal vehicle. I have run the service bay tests with the same results as at idle/driving. It cycles up and down, and of course fails the test.

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Michael Owner/Technician
Texas
Michael Default
 

What I learned when it fails, it was on deceleration when I tested a Pontiac for a week and several aftermarket Cat's when the DTC set.

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Juan Technician
California
Juan Default
   

I've seen certain GMs that don't like aftermarket Cats. Have to have OEM, my car is one of them.

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Bill Diagnostician
Pennsylvania
Bill Default
 

We had the same car but 09 in the shop. Low miles as well. New o2 sensors, New cat X2 went to dealer for reprogramming and finally put the GM cat on it. Readings were no different with either cat so not sure how the computer can see the difference. OEM got you by the balls.

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Eric Owner/Technician
Wisconsin
Eric Default
 

Bill, Why is it the OEM's fault the aftermarket catalytic converter manufactures make cheap products that don't meet specifications? If the aftermarket cat was built correctly it would pass emissions and the PCM wouldn't set failed cat codes. The PCM is in charge of monitoring the exhaust and if the PCM says it isn't clean enough then it is the fault of the aftermarket cat (as long as the rest…

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Paul Mechanic
Illinois
Paul Default
 

Back in the I/M 240 days when I use to sell a lot of cats, the documents/warranties that came with stipulated 25k or 2 years on performance of I remember correctly, and I believe it hasn't change. These days if I'm buying a cat, I specify that I'm replacing cat due to emissions and keeping light out. If seller cannot guarantee, I look elsewhere. I don't bother even looking with California…

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Lewis Technician
California
Lewis Default
 

NO YOU ARE WRONG !!!! the pcm runs many different tests that YOU are not privy to . this is how they are DESIGNED ! THEY DON'T HAVE YOU , YOU DON'T UNDERSTAND HOW IT WORKS . do you understand fuel control ? do you actually understand how this all works ? because you cannot get around certain design parameters , doesn't mean they “have you” . don't spread information like this when you have…

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Bill Diagnostician
Pennsylvania
Bill Default
 

Thank you for the kind words. Please don't assume what I do and don't know though. Have a great day anyway.

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Rudy Technician
California
Rudy Default
 

Id replace with an OE cat before chasing my tail. Its a no brainer…

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Keith Diagnostician
Oklahoma
Keith Default
 

Reading that your trims are good, and the short frequency that the P0420 sets, I would perform a snap throttle test while monitoring the O2 voltages… post the captures here please. I would like to inspect for a small exhaust anomaly I frequently see that can cause this.

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William Diagnostician
Virginia
William Default
 

I will get that done and posted. Thanks!!

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William Diagnostician
Virginia
William Default
 

Here is a capture of two throttle snaps including blown up saves. Taken with just these 4 pids selected.

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LaMont Owner/Technician
Ohio
LaMont Default
 

We had the same scenario on a Lucerne. Had to replace the aftermarket cat with OE. I'm sure the aftermarket company will say it should work, but the proof is in the pudding.

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Michael Owner/Technician
Texas
Michael Default
 

I have replaced many new aftermarket cats with OEM for the same issue on GM's. The Pontiac G5 was the most common one.

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Lewis Technician
California
Lewis Default
 

you CANNOT use an aftermarket cat…period . look at enabling conditions for the code to set and follow the bouncing ball . VERIFY no exhaust leaks . even a tiny leak will throw extra air into the system . VERIFY fuel control . if the above conditions are met , replace with o.e. or ca. certified .

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Scott Owner
California
Scott Default
   

Hi William, I'll throw my 3 cents in. Inflation... In absence of any calibration updates for P0420 - which there are not for this vehicle. Exhaust leaks... It gets a converter. I realize you've already done this, although you've used aftermarket versions. I haven't gotten into the habit of looking at HO2S data to determine whether or not it gets a cat, as I'm of the opinion with extended…

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Paul Mechanic
Illinois
Paul Default
 

William, this topic at this point, in my opinion, has turned into more of a informational/educational post, rather than the diagnostic question this started out as, please indulge me. I apologize in advance if I offend you in away about your knowledge and understanding of how the monitor runs and the code is set. I'm going of off past experience with testing GM cats in the bay or during road…

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William Diagnostician
Virginia
William Default
 

Thanks for this information. I will give it a try soon as I can and let you know what happened. Thanks!!

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William Diagnostician
Virginia
William Default
   

Here is the data and some screen saves from running the Service Bay CAT Test as you described. It worked great. Still looks like a BAD CAT to me. Let me know what you all think. Thanks!!

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Stephen Technician
Tennessee
Stephen Default
 

All of your posted pictures show a front O2 that is dithering too much. The swings are not full and not stable. I don’t think that it has air/fuel control. The sensor is 9 months old, I don’t discount that an OE cat is better but, even a poor knockoff should last a little while. A typical bad cat will cause the rear O2 to almost mimic the front. Yours is not.

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William Diagnostician
Virginia
William Default
 

Wondered about that myself. But have seen others doing it with no issues. Fuel trims are perfect. I may try a new one anyway and see what happens. Thanks!!

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Jim Educator
California
Jim Default
 

Hi William ! I think Scott gave you the best advice. Check for what the ECM cannot and then trust the onboard diagnostic I believe is the safest way to go. Will you have anomalies from time to time? Sure. But lets not pretend we can perform the monitor test better than the ECM can. You know… I wonder if, by chance, the calibrator looks at the trim control/limits when the catalyst monitor…

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Jim Educator
California
Jim Default
 

Stephen Did you measure the frequency of the sensor's switch time? A rough number is 200ms per frame. You see noise? This is zoomed out

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Stephen Technician
Tennessee
Stephen Default
 

Jim, I don’t think that it’s noise. I am looking at the front O2’s pattern at idle. I expected it to be even and consistent. Some swings were not full(.1-.9) and there are places where it holds hi for short periods. On the scan share it shows up at higher RPMs too, not as much, but, all of the idle times have it. I’m not disputing that the cat is bad and I’m not blaming the O2 sensor so much as…

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Jim Educator
California
Jim Default
 

Hi Stephen I think this is why we need to look at these more often and in context. Is this one concerning as well? scanshare​.​io/share/zJc_9cPv… It shouldn't be, it is from my low mileage 2018 company vehicle. The data stream is updated at the same rate as the OP's vehicle. That is not to say that every PID data point is updated in every packet though. That is why we need to…

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Stephen Technician
Tennessee
Stephen Default
 

Jim, Good Evening, I think that I understand what you are saying. Maybe the trees are blocking my view of the forest. My concern is that the front O2 is not stable even though engine rpm and load are. On your truck I see a similar though less pronounced pattern but, on your truck the RPM is not constant and only low idle for a short period. During that short period the O2 crosscounts evenly. The…

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William Diagnostician
Virginia
William Default
 

I also am not happy with the front O2 pattern. I will be replacing it later this week. I will post those patterns when I do. Thanks!!

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Jim Educator
California
Jim Default
 

Stephen and William Do you think it is possible… if not very likely, that the monitor takes O2 response into account? Perhaps even where in the trim control window the engine is operating? If we, as technicians, see an accurate O2 response as essential for the monitor, would we not think that a calibrator takes that into account as well (directly or indirectly)? Would it not be prudent for a…

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Scott Owner
California
Scott Default
 

Hi Jim, Compared to some other DTC's that I see, there are some that are just a slam dunk. A few that pop into mind - P0420/P0430 & P0128. I find it amazing the amount of time that is sometimes spent second guessing the manufacturers and looking for any reason other than the one that it is. I wish I could find some of my captures that if you were to look at them you'd say the converter was…

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Ernie Technician
New York
Ernie Default
 

Hey William. You did not mention what brand of AM Cat you used, I am assuming it is a Walker. If so, please take the time to read the Warranty info in the back of their catalog. They explicitly [apologetically] state thier Cats DO NOT contain the same amount of material as the OE's, but are a ‘economical’ alternative. I will let you be the judge if you think that statement is fair. Hey, they at…

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William Diagnostician
Virginia
William Update
 

I have replaced the front O2 with a AC Delco sensor, cleared the codes and monitors, and driven vehicle. All the monitors ran, and only the Cat failed. i performed the service bay test, and the results were the same: the Cat test failed during the test. Also the front and rear O2 patterns stayed exactly the same as before. I will be contacting Walker’s support people tomorrow morning about…

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LaMont Owner/Technician
Ohio
LaMont Default
 

Anxious to find out if Walker actually admits to having a problem. In my experience, they are never at fault when you have a problem.

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Jim Educator
California
Jim Default
 

Perhaps that is because they don't have to pass an independent lab test or meet the original certification levels to be sold legally in most states. I don't think that catalyst would make the CARB cut. Microsoft PowerPoint - AMCC Presentation Final 120814​.​pptx (maryland​.​gov) It is being assumed (for the sake of cost) that you can cut down on the precious metals…

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Phil Technician
Massachusetts
Phil Default
 

Walker will deny, deny, deny and state that it passes their testing-which does not include keeping the MIL off. Walker doesn't even list a CARB catalyst for this application which means the cat you have purchased only has to pass a 5 gas test that Walker self certifies that the unit passes. There is no independent testing group that tests catalysts before they are allowed to be sold. A quick…

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William Diagnostician
Virginia
William Update
 

I have talked to Walker about this issue. Was a good call, much better than I expected. they have 2 tests they want done, so i will perform them and report back. Thanks!!!

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William Diagnostician
Virginia
William Update
 

Here are the 2 tests they wanted run. They wanted a good drive under varied conditions showing misfire/history counters. I also reran the service bay test and timed the front/rear switch times. Research shows GM wants much longer time between O2’s switching during the test. Mine not so much. I’ll keep you informed of the progress. Thanks!!

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Paul Mechanic
Illinois
Paul Default
 

What are those humps just after the 15000 mark on the bank 2 cylinders? Your cat capture, how did you determine elapsed time?

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William Diagnostician
Virginia
William Default
 

The humps are a misfire indication where the counter clicked up. Those are the only ones on the entire drive of about 20 minutes. i timed the O2 response with second hand of my watch.

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Paul Mechanic
Illinois
Paul Default
 

Ok. Did you happen to capture fuel trims- if they are tight, gives you more ammunition that the engine is not issue. Also, the fact that your O2 signals show no hash (not sure if scan data graphed registers misfire like a scope would) also supports it's not an engine management issue. Good luck.

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Phil Technician
Massachusetts
Phil Default
 

Update??? Just wondering what Walker has to say about the situation and what they say about the information you provided them.

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William Diagnostician
Virginia
William Default
 

I have hit a wall with them. They want a 5 gas readout, but our analyzer will not calibrate and must be repaired. I’m not sure why since the fuel trims are great under all driving conditions. I am currently pricing a OE converter to install soon. I’ll post the results once installed.

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Scott Owner
California
Scott Default
 

Hi William, Can I ask why they're asking for that? This is a rhetorical question as I know the reason why is they don't want to pay, but...What will this prove? If the I/M readiness flags are complete and there is a P0420 DTC stored, their catalyst is defective. California stopped running vehicles on the dyno and testing tailpipe emissions on 2000 and later MY's due to the testing run by the…

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William Diagnostician
Virginia
William Default
 

Not sure what’s they expect to see since the PCM can only see O2, not the other 4 gasses. If the mixture was off they would have spotted it and adjusted trim accordingly.

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Paul Mechanic
Illinois
Paul Default
 

Don't forget to pin them on why an oe cat satisfies the PCM, but not the Walker? Maybe find a shop nearby that has a functioning five gasser to prove to them it's their product and not your car.

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Michael Owner/Technician
Texas
Michael Default
 

I would love to see an aftermarket reading versus and OEM reading. We haven't had a gas analyzer in years.

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Phil Technician
Massachusetts
Phil Default
 

Well that is unfortunate, but not unexpected. This has been their M.O. for quite some time. The 5 gas readout is meaningless if the part won't keep the MIL off. Just one more way that they throw up a roadblock to resolving the actual situation of an inferior quality part. As long as your original catalyst wasn't killed by misfires or an engine passing oil or coolant into the exhaust then you…

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William Diagnostician
Virginia
William Resolution
 

Well the Buick is fixed with a factory CAT. Rear O2 stays nice and flat, and the PCM passed the tests on the first try. Attached are some screen shots of the service bay test and the mode 6 results after the test was run. Hopefully this helps if anyone ever needs to see what a factory CAT test looks like. Thanks for all the help and suggestions!!

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Paul Mechanic
Illinois
Paul Default
 

What was Walker's response?

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Michael Owner/Technician
Texas
Michael Default
 

A response would be nice, because I have replaced so many aftermarket cats without ever knowing why an OEM was better. The only thing I ever noticed was that a CEL would set on deceleration after cruising for a while, which I believe was when the cat was tested. I will have to look at the before and after on a PC later.

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William Diagnostician
Virginia
William Default
 

i sent them the same info. No response yet. Has been a week now with no codes.

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