Unrelenting P0420 remains after catalytic converter, O2 sensors, and other parts replaced and modified

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nato
Posts: 16
Joined: Sat Aug 29, 2020 8:07 pm

Unrelenting P0420 remains after catalytic converter, O2 sensors, and other parts replaced and modified

Post by nato » Mon Apr 12, 2021 10:02 am

edit: Replaced catalytic converter, and a number of other parts before fixing it with spark plug non-foulers. The fix lasted two weeks which was more than enough time to pass smog. The thread subject was also modified more than once. See subsequent posts in this thread for more details.

Hi,

I have a 2005 Matrix XR with 271K miles that has a CEL that I need extinguished so I can get it to pass emission/inspection. I've checked it multiple times (and reset the computer) and it is usually just a P0420. Today I checked and also saw a P0171.

Details that may or may not be relevant:
  • About two years ago I was in the same situation of having a P0420 and needing it extinguished. I was thinking at the time I wouldn't have the car more than 6 more months so I replaced the catalytic converter with a $120 version from ebay and reset the computer. I drove it around until it completed the drive cycles (maybe 1 or 2 days) and then took it immediately to get emissions/inspection. It passed, but less than a week later the light came on, and you guessed it: P0420.
  • I push this car hard, and occasionally (for example, at 80mph with the gas pedal floored and engine wound kinda high) there'll be this hesitation which my only slightly-knowledgeable head says is detonation and/or the computer fiddling with the timing as it detects detonation/lean. That could be related to the P0171. This is just a slightly-educated guess, and what I'm getting at here is that the P0171 may be a secondary result of whatever is causing the P0420 (remember, the P0171 isn't always present, but the P0420 is always present).
  • I get probably one or two drops of oil when i park it and top off the oil with a quart probably every 2 months. I'm guessing with the small amount of oil that it leaks that I might be burning some oil too, but not enough to be obvious from the exhaust (maybe a year and a half ago I checked and had good compression in every cylinder). I include this because my understanding is that oil can foul O2 sensors and cats.
  • I can occasionally smell antifreeze on a hot day after I park it and every two to three months I add a bit of coolant to top it off. I include this because my understanding is that antifreeze can foul O2 sensors and cats.
  • My $40 code reader also has "live data" capability. So I can see the voltage levels of both O2 sensors. However, ... I don't know what they should be or how they should bounce around, and also the values only update about once a second and it isn't clear to me if that's fast enough to be useful. Further, it just gives numbers and doesn't plot--not sure if that's what's really needed to useful or not.
This coming weekend I want to take care of this. My goal is the same as last time: get emissions/inspection passed so I can drive it for 6 more months. I have another cat ($140 amazon, but actually has positive reviews) and both O2 sensors.

I'm thinking of just replacing the O2 sensors and then trying to complete the drive cycle to see if that fixes the issue (pretty sure the P0420 gets set in like 5 minutes, well before all drive cycles are complete). And then if that doesn't take care of the problem then replacing the 2 year old catalytic converter.

Is this a reasonable approach? Too much, too little? I don't currently have an extra car for trips to Autozone/O'Reilly's/Advance Auto and want to have parts I might need on-hand. Anything I might be missing?

Anybody know what I should be seeing with the O2 sensor voltage levels? Or know of a good source you can point me to on that?

Thanks so much,
nato
Last edited by nato on Mon May 03, 2021 6:44 am, edited 4 times in total.

zbyers
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Location: Sheffield, Pennsylvania

Re: Plan for solving P0420?

Post by zbyers » Mon Apr 12, 2021 11:28 am

P0420 = bad cat 99% of the time. The code itself says the o2 sensor is working, as it detected the converter is below efficiency thresholds.

P0171 = intake manifold gasket most of the time...could be a vacuum leak, but usually intake manifold gasket.

Replacing o2 sensors is pointless.

If you are burning oil, it can cause your cat to go bad.

The first o2 sensor should be a nice wave. The second one should remain relatively flat, until you step on the throttle, and then you should start seeing it wave too.
'74 Thing, '79 El Camino, '83 VW Vanagon DoKa '91 Vanagon,
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nato
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Joined: Sat Aug 29, 2020 8:07 pm

Re: Plan for solving P0420?

Post by nato » Fri Apr 16, 2021 9:02 pm

zbyers wrote:
Mon Apr 12, 2021 11:28 am
P0420 = bad cat 99% of the time. The code itself says the o2 sensor is working, as it detected the converter is below efficiency thresholds.
OK, I just got done replacing the cat. I cleared the codes and took it for a test drive. Less than 5 minutes in and I have a pending P0420. Anybody got suggestions for the next most likely culprit?
zbyers wrote:
Mon Apr 12, 2021 11:28 am
P0171 = intake manifold gasket most of the time...could be a vacuum leak, but usually intake manifold gasket.
OK, that's believable. I plan to leave it be for now and not bother with this unless that code comes back.
zbyers wrote:
Mon Apr 12, 2021 11:28 am
The first o2 sensor should be a nice wave. The second one should remain relatively flat, until you step on the throttle, and then you should start seeing it wave too.
So a "wave", as in a sinusoidal pattern? Or if not really pretty, at least going up and down continually?

What sort of bias/offset should I expect for each of them?

How about ballpark minimum and maximum values?

And what type of period are we talking about--that is, how often does the cycle repeat?

Will I even be able to make this out with the reader I described that updates the voltage values about once a second?

Thank you all in advance for your wisdom and taking the time to respond,
nato

nato
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Re: P0420 after catalytic converter replacement

Post by nato » Sat Apr 17, 2021 8:19 am

OK, so it turns out that my code reader does have the option to plot a graph of a single parameter. I spent some time this morning getting plots of the O2 sensors. I'd appreciate it if someone could tell me what these plots suggest, although admittedly I'll most likely have replaced the sensors before I get a reply as I'm anxious since my car currently has expired tags.

The following is the upstream sensor while accelerating at a moderate rate:
upstream_acceleration.jpg
upstream_acceleration.jpg (313.51 KiB) Viewed 157 times
The following is the downstream sensor while accelerating at a moderate rate:
downstream_accelerate.jpg
downstream_accelerate.jpg (473.84 KiB) Viewed 157 times
The following is the upstream sensor. The first third or so is idling at a stop sign. The second third is accelerating at a moderate rate. The third third is coasting.
upstream_idle_acceleration_coast.jpg
upstream_idle_acceleration_coast.jpg (419.16 KiB) Viewed 157 times
The following is the downstream sensor. The first third or so is idling at a stop sign. The second third is accelerating at a moderate rate. The third third is coasting.
downstream_idle_accelerate_coast.jpg
downstream_idle_accelerate_coast.jpg (374.74 KiB) Viewed 157 times
Thanks so much in advance for any other suggestions on getting my car fixed,
nato

nato
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Joined: Sat Aug 29, 2020 8:07 pm

Re: P0420 after catalytic converter replacement

Post by nato » Sat Apr 17, 2021 7:58 pm

tldr: I replaced the catalytic converter, both O2 sensors, intake manifold gasket, and spark plugs. I still have P0420. Please help!

I replaced the upstream O2 sensor, reset the computer, and went for a test drive. The P0420 appears in the pending list after 5 minutes of driving.

I replaced the downstream O2 sensor, reset the computer, and went for a test drive. The P0420 appears in the pending list after 5 minutes of driving.

I believe these were the original (271K miles) sensors. Upstream sensor is on the left; downstream sensor on the right:
O2Sensors.jpg
O2Sensors.jpg (1000.53 KiB) Viewed 146 times
I already had the intake manifold gasket on hand and figured it wouldn't hurt to replace it since it's gotta be done sometime anyways. Maybe it's the culprit? ... Nope! :( While I was at it I replaced the spark plugs. I reset the computer and went for a test drive. The P0420 appears in the pending list after 5 minutes of driving.

Anybody have any ideas I should consider for my next step to fix the P0420?
intake_manifold_gasket.jpg
intake_manifold_gasket.jpg (720.71 KiB) Viewed 146 times
Spark plugs in cylinder order (left is cylinder 1; right is cylinder 4):
spark_plugs.jpg
spark_plugs.jpg (862.56 KiB) Viewed 146 times

Mark
Posts: 535
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Location: Cheyenne, WY

Re: P0420 remains after catalytic converter, O2 sensors, and other parts replaced

Post by Mark » Sun Apr 18, 2021 8:58 am

https://www.autozone.com/ignition-tune- ... 720805_0_0

That's what I've done on Corolla and one Vibe, no more P0420.
'04 Base Frosty, '08 Base Stealth, '05 Base Satellite

nato
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Re: P0420 remains after catalytic converter, O2 sensors, and other parts replaced

Post by nato » Sun Apr 18, 2021 9:33 am

Mark wrote:
Sun Apr 18, 2021 8:58 am
https://www.autozone.com/ignition-tune- ... 720805_0_0

That's what I've done on Corolla and one Vibe, no more P0420.
Thank you so much for the reply! I'll give this a try.

Edit: OK, I see elsewhere in the forum and a reference to another forum on how to do this. Looks like the preferred way is to use two extenders daisy-chained together. The extender closer to the sensor has its bottom (half opposite from sensor) drilled out to allow for the sensor end to fit. This is to be used for the downstream sensor and not the upstream sensor. Anybody feel free to correct me if you have other information or if I misunderstood

Thanks again!
nato

Mark
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Joined: Sun Nov 02, 2008 7:26 am
Location: Cheyenne, WY

Re: P0420 remains after catalytic converter, O2 sensors, and other parts replaced

Post by Mark » Sun Apr 18, 2021 1:23 pm

Nope, you got it right. This probably won't pass a visual inspection, but if they are just looking for codes it will solve your problem.

I've found that I have to start with a small drill bit and gradually work my way up to 1/2".
'04 Base Frosty, '08 Base Stealth, '05 Base Satellite

nato
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Re: P0420 remains after catalytic converter, O2 sensors, and other parts replaced

Post by nato » Sun Apr 18, 2021 9:01 pm

Whoa! The non-fouler trick may have "fixed" the car.

I've driven the car three times so far, for at least 40 minutes total drive time. No set codes, no pending codes, catalytic converter and O2 sensor drive cycle monitors are showing ready.

The evaporative system monitor is the only thing missing, and I read something yesterday that suggests I might even be able to pass emission test without it here (in Missouri). I plan to take it to get tested as soon as possible and will update this thread with the results.

For future reference I've included information and images following.

I believe the link to the non-fouler above is for non-fouler that won't work for this O2 sensor. The link points to the Dorman 42006 which lists a 14mm thread size (outside diameter of theads). I have the Dorman 42009 which has 18mm threads and fits my catalytic converter assembly's pipe hole and O2 sensor after one of the non-foulers was modified.
dorman42009.jpg
dorman42009.jpg (91.76 KiB) Viewed 123 times
I drilled the hole out of one of the non-foulers. There's a small hole and then the inside diameter of the body of the non-fouler is larger than the small hole--both get drilled as the drilled hole was larger than the inside diameter of the body. I used oil to help the bits cut the steel and I used 5 or 6 bits ramping the size up a bit each time. I used a wrench to center the non-fouler on the drill press plate
drilling.jpg
drilling.jpg (195.7 KiB) Viewed 123 times
The unthreaded portion of the O2 sensor that slips into this drilled out portion of the non-fouler has a diameter that is somewhere under 0.48". My final, largest bit used had a diameter of about 0.48".
drilled.jpg
drilled.jpg (145.22 KiB) Viewed 123 times
Threading an O2 sensor into the first non-fouler one can see that the tip of the O2 sensor comes nearly flush with the end of the non-fouler.
inserted_nonfouler.jpg
inserted_nonfouler.jpg (149.87 KiB) Viewed 123 times
I threaded the modified non-fouler into the other unmodified non-fouler.
assembled_sensor.jpg
assembled_sensor.jpg (175.3 KiB) Viewed 123 times
I saw elsewhere that suggested some applications of this method doesn't work using both non-foulers and suggest not using adding the unmodified non-fouler onto the assembly that'll be screwed into the catalytic converter assembly's pipe hole. I planned to try removing the additional unmodified non-fouler and use only the modified one if the two non-fouler application didn't prevent the P0420 codes. We'll see how this goes.

Note that I used the crush washer/metal gasket on the end of this assembly when screwing it into the catalytic converter assembly's pipe hole and the crush washer/metal gasket end up being crushed non-uniformly. If I wasn't so lazy I would've backed the assembly out and used the assembly without one. In the end it probably won't make much difference either way.

Thanks again!
nato
Last edited by nato on Mon Apr 19, 2021 2:10 pm, edited 1 time in total.

Mark
Posts: 535
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Location: Cheyenne, WY

Re: P0420 remains after catalytic converter, O2 sensors, and other parts replaced

Post by Mark » Mon Apr 19, 2021 1:32 pm

nato wrote:
Sun Apr 18, 2021 9:01 pm
Whoa! The non-fouler trick may have "fixed" the car.

I believe the link to the non-fouler above is for non-fouler that won't work for this O2 sensor. The link points to the Dorman 42006 which lists a 14mm thread size (outside diameter of theads). I have the Dorman 42009 which has 18mm threads and fits my catalytic converter assembly's pipe hole and O2 sensor after one of the non-foulers was modified.

I used a wrench to center the non-fouler on the drill press plate
Sorry, I couldn't remember what size I used, so I just grabbed the first one when I searched on AutoZone.

A drill press would have made it so much easier and faster. Glad to hear it worked for you, it's a lot cheaper than a new cat and O2 sensors, for sure.
Last edited by Mark on Mon Apr 19, 2021 6:39 pm, edited 1 time in total.
'04 Base Frosty, '08 Base Stealth, '05 Base Satellite

nato
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Re: P0420 remains after catalytic converter, O2 sensors, and other parts replaced

Post by nato » Mon Apr 19, 2021 5:56 pm

OK, my P0420 issue is finally resolved. All OBD2 readiness monitors passed by this morning, I was able to get an official smog test pass today and get the tag to prove I payed my taxes.

See the previous post for details, but the highlights are:
1. Replacing the catalytic converter, both oxygen sensors, intake manifold gasket (primarily due to another code that I'd dealt with), and the spark plugs didn't fix the P0420 issue.
2. After doing all of the above I added two spark plug non-foulers (after modifying one of them) between the catalytic converter assembly's pipe and the downstream oxygen sensor. This fixed the P0420 issue.


So just shooting from the hip here: By extending the distance between the oxygen sensor and the catalytic converter itself the speed at which the sensor measures the temperatures is probably slowed down. It might also be attenuated as the ambient air might keep it from reaching as high of temperatures as it was before. If this is what's going on then it would seem that one or more of the following is true:
  • The OBD2 catalytic converter monitor is improperly tuned, and/or
  • The previous two catalytic converters I've had along with the current catalytic converter are all out of spec, and/or
  • The previous oxygen sensor and current oxygen sensor are both out of spec.
The last two reasons seem quite unlikely as they rely on having multiple failures, including failures of brand new parts. The fact that a number of folks have used the non-fouler installation method successfully to be rid of the P0420 error seems to corroborate the hypothesis of an incorrectly tuned OBD2 system. Of course, there's no way to really know for sure as there's no way to know what's going on inside the ECU.

IANAL, but I'm pretty sure what I did was illegal. As far as I understand, it'd be literally impossible to have legally fixed the issue so I could renew the registration (which is required to legally drive the car). I could spend some time now ranting about the injustice of a federal regulation that levies expensive requirements on the little guy while the regulation is configured such that it disallows independent verification (because of proprietary/closed source software and corporate "secrets"). I could also rant how the same baloney allowed Volkswagen to get away with their emissions scandal for at lease 17 years, and how others in the know have revealed that Volkswagen isn't the only manufacturer that cheats on emissions, and that it's just that they're the only one that's been caught as nearly every other manufacturer pulls the same sorts of baloney. I could complain about it, but I'm not going to.

Maybe I'm just full of baloney.

I'd like to offer huge thanks to Mark and zbyers for their help!
nato

nato
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Re: SOLVED: P0420 remains after catalytic converter, O2 sensors, and other parts replaced

Post by nato » Mon May 03, 2021 6:42 am

OK, here I am two weeks later and the code is back. Admittedly, I was pushing it really hard--accelerating really hard up to 90mph driving on the interstate this morning. At some point I was at a stop light and noticed the engine light was on. I had the scanner in the glove compartment; connected it and saw I had a P0420 active code and a P0420 pending code.

<sigh>

Mark
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Re: Unrelenting P0420 remains after catalytic converter, O2 sensors, and other parts replaced and modified

Post by Mark » Mon May 03, 2021 4:22 pm

Well that sucks. Hope you're right and it was just due to pushing it hard. I would just clear it and see how it goes, but the important thing is that you passed emissions. So at this point the CEL is mainly just an annoyance.
'04 Base Frosty, '08 Base Stealth, '05 Base Satellite

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