I have seen suggestions to buy a kit to turn off VCM.

YES.
the garbage VCM system fouls plugs and catalytic converters.

 

Try a bottle of redline in 1/2 a tank of gas - - 30 min highway drive.

 

You change the O2 sensors first because they are often the problem, despite what the code says. It's an inexpensive way to start addressing the trouble code. And even if you end up having to replace the cat, you're going to want new O2 sensors in there afterwards anyway.

 

Thanks, what are your thoughts on just replacing the 2 oxygen sensors for the reported Bank instead of replacing all 4.

 

Either load the parts cannon, or ...

If you're doing it yourself, willing to experiment and think, and want to spend extra effort to avoid unneccesarily replacing parts ...

The algorithm to trigger the P0420 is notoriously inaccurate. First thing I would do is to reset the code, log the date and mileage, and see how long it will take to come back.

Once (and if) it comes back, if you want to figure out whether it is the cat vs. the O2 sensors, you could try swapping them bank1 to bank2, and see if the error code(s) move.

(not sure exactly how much of this is accurate on every car, but ...)
Very roughly speaking, the way the P0420 code happens is that the downstream O2 sensor signal is bouncing up and down rather than staying level in the middle. This indicates a lack of reactions going on in the cat. Problem is that this algorithm is very indirect and therefore inaccurate as a fault detector.

One solution is to use an extender device installed with the downstream O2 sensor. That creates a small, accumulating cavity of exhaust gases, which allows them to accumulate and mix, naturally damping out the dynamic variations, and keeping the fault detection system happy. If your system is actually good, and the fault triggering is bogus, this will take care of the code; and if the system is bad, a smog sniffer will detect bad emissions out the tailpipe, even with no code.

Also, in general, often times cat problems are caused by engine issues that, for example, send too many unburned hydrocarbons through the cat, making it too hot, degrading the honeycomb mesh, causing the catalyst materials to crumble away and not do their job.

So be on high alert for engine problems, which your O2 sensors should show you. If there is anything weird with what the O2 sensors show, and once you have confirmed they are good sensors, I'd work on that - and do it before worrying about the cat.

The P0420 code indicates a problem that may or may not be real. But it does not affect anything at all about how the engine runs. The only harm it can do is to cause you to fail a smog check, and to maybe miss another code that pops up while you are ignoring the check engine light.

 

Either load the parts cannon, or ...

If you're doing it yourself, willing to experiment and think, and want to spend extra effort to avoid unneccesarily replacing parts ...

The algorithm to trigger the P0420 is notoriously inaccurate. First thing I would do is to reset the code, log the date and mileage, and see how long it will take to come back.

Once (and if) it comes back, if you want to figure out whether it is the cat vs. the O2 sensors, you could try swapping them bank1 to bank2, and see if the error code(s) move.

(not sure exactly how much of this is accurate on every car, but ...)
Very roughly speaking, the way the P0420 code happens is that the downstream O2 sensor signal is bouncing up and down rather than staying level in the middle. This indicates a lack of reactions going on in the cat. Problem is that this algorithm is very indirect and therefore inaccurate as a fault detector.

One solution is to use an extender device installed with the downstream O2 sensor. That creates a small, accumulating cavity of exhaust gases, which allows them to accumulate and mix, naturally damping out the dynamic variations, and keeping the fault detection system happy. If your system is actually good, and the fault triggering is bogus, this will take care of the code; and if the system is bad, a smog sniffer will detect bad emissions out the tailpipe, even with no code.

Also, in general, often times cat problems are caused by engine issues that, for example, send too many unburned hydrocarbons through the cat, making it too hot, degrading the honeycomb mesh, causing the catalyst materials to crumble away and not do their job.

So be on high alert for engine problems, which your O2 sensors should show you. If there is anything weird with what the O2 sensors show, and once you have confirmed they are good sensors, I'd work on that - and do it before worrying about the cat.

Solid advice. The OP should absolutely be disabling VCM (much as he doesn't seem to want to hear that right now) as a means of making sure the engine is healthy. It's not the only thing to look at when it comes to engine condition but it is at the top of the list. Once you know the engine is OK, then you look into the O2 sensors as the next possible culprit (and cheapest possible parts cannon fodder) and only at the end of the diagnostic process should you conclude the catalytic converters should be replaced. Ascending order of complexity and cost.

 

Take apart and re-connect the O2 wire harnesses...could be a loose connection. I doubt it but you never know, and it can't hurt to cross this off the troubleshooting list. Also could be an exhaust leak.

 

I'd also suggest making a note of it, resetting the code, and seeing if it comes back. On our 05, we would set a P0420 intermittently until a code specific to the O2 sensor popped up - replaced it and all was well until we started throwing P0430, and followed the same procedure eventually getting an O2 specific code and replacing the sensor (first one was at 173k miles, second at 200k miles. A year or two after that, we would sometimes throw a P0420 or P0430, oddly only on really windy days.

 

Yes, P0420 likely means your bank 1 (rear) cat is toast. It could be due to skewed O2 sensors or could be due to oil burning due to VCM. I'd pull the rear bank of plugs and see what they look like. They are are due at 105K anyway so probably just replace them if they haven't been replaced already. Look for signs of oil fouling and compare them to the plugs from cylinders 5 & 6. Cylinder 4 on the front bank is also a VCM cylinder so it may be oil fouled, as well.

Since your cat needs replacing you wouldn't put a new cat on with old O2's so that's why I recommend just replacing the O2's first. You're out nothing additional if it doesn't fix it. If it does help, you've put off the costly cat repair...for a while, anyway. You can try replacing both O2's on bank 1 and see if there is an improvement in the fuel trims. In the grand scheme of things, the O2's are only really good for 100-130K miles anyway

 

I replaced all four at around 180K miles. I was getting the P0420 code as well. The sensors alone got me to about 210K miles. Eventually the code returned more often. I did use some 1” spacers on the downstream sensors and have not seen the code since. Fools computer into thinking the downstream temperatures are lower than they really are. Passed my latest emissions check so it was marginal anyway.

New cats are potentially still in my future but for now seems to be fine.

BTW the VCM inhibit should not be optional. Mileage hit is minor as I still can get 30mpg with a light foot and flat roads. My last plug change at 215K miles showed perfect operation.

 

I would make sure that the cat REALLY needs to be replaced before replacing it. Not the same car, but I had the P0420 on my Toyota Camry at around 70K miles. Mechanic said I needed a new cat, or could be O2 sensors. I ended up resetting the code to pass my bi-annual emissions inspection, not fixing anything, but the CEL always came back. Two years later, did the same thing again to pass inspection. Then around 90K, the car developed an exhaust leak. After that was patched, miraculously, the code never came back, and I'm up to 106K now.

 

Maybe the bulb just gave up. =)

 

Exhaust leaks can definitely cause a P0420 or P0430 code.

 

An OBDII reader will help you monitor O2 sensor outputs. Even the cheap Bluetooth/WiFi versions work.

The extenders for the sensors are marketed on EBAY and Amazon. Here’s an example.

 

The trims are a bit high. The P0420 isn't the cause but could be the result of whatever is causing the high fuel trims.

As for the O2 sensor extenders, those are technically illegal to use for that so if your state requires inspections you could run into an issue with them. You're essentially just masking the problem with them anyway and you could end up with a P0430, as well.

 

2012 Honda Odyssey EX-L. 108,270 Miles. Houston, TX. (but the majority of miles were in Southern CA and MA)

I had a CEL, using my BlueDriver Bluetooth OBDII Scanner, I pulled Code P0420 - "Catalyst System Efficiency Below Threshold Bank 1"

After reading a bunch of posts on this forum, the first suggestion seems to be to replace all 4 oxygen sensors even though the problem is likely the Catalytic Converter. My questions are below:

1. Can someone confirm that the 4 oxygen sensors I should buy are the ones displayed below?
2. Why is the suggestion to replace all 4 oxygen sensors instead of just replacing the 2 oxygen sensors for Bank 1? It seems like a waste of money to replace the Bank 2 oxygen sensors since I did not get a P0430.
3. After replacing the oxygen sensors and clearing my codes, if P0420 returns is the next step to replace the catalytic converter? It doesn't seem right that my cat converter has died after only 108K miles.
4. If I have to replace the catalytic converter, do I just replace the rear one? I read somewhere that there are 3 cat converters which I don't understand since I thought there was just 1 cat per bank.
5. I have seen suggestions to buy a kit to turn off VCM. I don't know much about this, I'll have to read more about it but it seems that there are strong opinions that if I don't turn off VCM, I will get repeat problems down the line. Not a question, just a statement since I know someone will mention turning off VCM.
6. I posted my Fuel Trim data below. Both banks seem really high. I'm not super smart on cars, can someone interpret what these values may suggest and how my P0420 code may be related? Thanks.

NTK 24435 {#36542R70A01, 36542R71L01} Info Downstream Rear; Bank 1 Sensor 2; OE MFR: NTK

NTK 24436 {#36532R70A01, 36532R71L01} Info Downstream Front; EX Model; EX-L Model; EXL Model; LX Model; Bank 2 Sensor 2; OE MFR: NTK

NTK 24385 {#36531R70A02, 36541R70A01} 5-Wire Wideband Info FitsUpstream Front; Bank 2 Sensor 1; OE MFR: NTK orUpstream Rear; Bank 1 Sensor 1; OE MFR: NTK

View attachment 156023

2012 Honda Odyssey EX-L. 108,270 Miles. Houston, TX. (but the majority of miles were in Southern CA and MA)

I had a CEL, using my BlueDriver Bluetooth OBDII Scanner, I pulled Code P0420 - "Catalyst System Efficiency Below Threshold Bank 1"

After reading a bunch of posts on this forum, the first suggestion seems to be to replace all 4 oxygen sensors even though the problem is likely the Catalytic Converter. My questions are below:

1. Can someone confirm that the 4 oxygen sensors I should buy are the ones displayed below?
2. Why is the suggestion to replace all 4 oxygen sensors instead of just replacing the 2 oxygen sensors for Bank 1? It seems like a waste of money to replace the Bank 2 oxygen sensors since I did not get a P0430.
3. After replacing the oxygen sensors and clearing my codes, if P0420 returns is the next step to replace the catalytic converter? It doesn't seem right that my cat converter has died after only 108K miles.
4. If I have to replace the catalytic converter, do I just replace the rear one? I read somewhere that there are 3 cat converters which I don't understand since I thought there was just 1 cat per bank.
5. I have seen suggestions to buy a kit to turn off VCM. I don't know much about this, I'll have to read more about it but it seems that there are strong opinions that if I don't turn off VCM, I will get repeat problems down the line. Not a question, just a statement since I know someone will mention turning off VCM.
6. I posted my Fuel Trim data below. Both banks seem really high. I'm not super smart on cars, can someone interpret what these values may suggest and how my P0420 code may be related? Thanks.

NTK 24435 {#36542R70A01, 36542R71L01} Info Downstream Rear; Bank 1 Sensor 2; OE MFR: NTK

NTK 24436 {#36532R70A01, 36532R71L01} Info Downstream Front; EX Model; EX-L Model; EXL Model; LX Model; Bank 2 Sensor 2; OE MFR: NTK

NTK 24385 {#36531R70A02, 36541R70A01} 5-Wire Wideband Info FitsUpstream Front; Bank 2 Sensor 1; OE MFR: NTK orUpstream Rear; Bank 1 Sensor 1; OE MFR: NTK

Most Honda Odyssey owners have a same experience with this P0420 code. It is bank 1, that on the rear side of the engineer. There are two o2 sensors. One of the top that connected to top or before the catalytic converter. This one can be replaced without go under your van. Another one is at the bottom where you have to go underneath. There are video available on you tube.

 

Any van with 100k miles or so should never have this code. Mine really didn’t start going off well into 180K. O2 sensors are only good for about 160 to 200k miles from what I’ve experienced. I did replace all four and monitored my trims and readings. It was clear my cat efficiency was just degrading with age. I elected to get this thing across the finish line to 300K miles by going with the spacers. Yes some folks frown on them. I’d of done the cat if I suspected it were breaking down or causing issues that way. In my case the cat was mechanically fine but marginal for emission monitoring.

I did recently pass my emissions test and if I ever fail it I certainly will replace the cats at that time. Just an economics decision on my part.

 

To the OP

remember you can still pass the smog if you clear the codes and get your green light catalyst set (drive cycle) before the P0420 comes back

In case there's pressure for a smog check

Then you can take your time figuring out how to fix the problem

...reading this thread I realize that on a 2006 Toyota Sienna that I work on, with 167k miles, that has intermittent P0420 and p0430... it may very well be the O2 sensors or an exhaust leak like mentioned above... I've tried cataclean to clean the catalytic converter which seemed to help. But there may have been confounding variables like mentioned in this thread that I was not thinking of.

 

Just had the P0420 code come up. 8000 miles after warranty repair for the fouling of plug 2. I haven't added the kit that bypasses the ECO feature yet (whole reason for the warranty).
What is the first step? add the kit to cancel the valve management or o2 sensors on bank 1. I have emissions test coming up so I need to work quickly

 

You can try two new O2 sensors but really the fix is a new catalytic converter.

Defeating the VCM is needed for long term reliability so it doesn't happen again but won't affect your catalytic converter in the short term.

 

Defeating the VCM is needed for long term reliability so it doesn't happen again but won't affect your catalytic converter in the short term.

Likely the cat (and/or O2 sensor) damage occurred during the first round of VCM issues. It would be worth trying new O2 sensor(s) and a product like Cataclean to see if that solves it. Along with a VCM defeating device, of course...

-Charlie

 

Yeah, trying some cataclean might be enough just to get you through inspection. Remember after you clear codes you need to drive it a few days before you can get it inspected. So, "trying" this or that can take some time.

 

Acetone is supposed to work as well but use at your own risk.

 

I used to put half cup of acetone in per full tank, as an experiment hypermiling on my daily commute with a 2001 hyundai accident
did that for over a year.

it made a minor difference in mileage with the obdii monitor tracking it.
showed 15-20 miles extra per tank, didnt care if it blew the thing up.
BTW it didnt...

 

I am having an intermittent P0420 code on my 05 EX as well. It has happened twice so far.

Three things to note:

1. it happened In the winter months--December and January. I bought the van--a 05 EX--in early October last year. When I drove it in October and November, no codes. I am in Southern Cali. The temps here in October and November are 80s, 70s, and even a few days of 90s.

2. it happened shortly after a complete fill up--16 to 18 gallons. It was either the same day or the next day, but within 25 miles of the fill up.

3. it happened in the morning, when the engine was just warming up or had just warmed up, and I was driving up a slight grade--1 or 2%

I looked up FSM about the causes to these codes, and it says bad gas could trigger these codes. I use 87 grade gas from Arco and another local gas station. Both are budget. Should I switch to a different brand and not budget?

Screenshot of the FSM, where it says bad gas could cause these codes:

I have also put in a bottle of Cataclean in the gas tank, drove till it was empty. Then filled up, after the fill up, on the same day, within the first 25 miles, the code came back!

I am planning on monitoring the Bank 1 O2 sensors voltages and fuel trims in the coming days.

 

I am having an intermittent P0420 code on my 05 EX as well. It has happened twice so far.

Three things to note:

1. it happened In the winter months--December and January. I bought the van--a 05 EX--in early October last year. When I drove it in October and November, no codes. I am in Southern Cali. The temps here in October and November are 80s, 70s, and even a few days of 90s.

2. it happened shortly after a complete fill up--16 to 18 gallons. It was either the same day or the next day, but within 25 miles of the fill up.

3. it happened in the morning, when the engine was just warming up or had just warmed up, and I was driving up a slight grade--1 or 2%

I looked up FSM about the causes to these codes, and it says bad gas could trigger these codes. I use 87 grade gas from Arco and another local gas station. Both are budget. Should I switch to a different brand and not budget?

Screenshot of the FSM, where it says bad gas could cause these codes:

View attachment 180870

I have also put in a bottle of Cataclean in the gas tank, drove till it was empty. Then filled up, after the fill up, on the same day, within the first 25 miles, the code came back!

I am planning on monitoring the Bank 1 O2 sensors voltages and fuel trims in the coming days.

I am having an intermittent P0420 code on my 05 EX as well. It has happened twice so far.

Three things to note:

1. it happened In the winter months--December and January. I bought the van--a 05 EX--in early October last year. When I drove it in October and November, no codes. I am in Southern Cali. The temps here in October and November are 80s, 70s, and even a few days of 90s.

2. it happened shortly after a complete fill up--16 to 18 gallons. It was either the same day or the next day, but within 25 miles of the fill up.

3. it happened in the morning, when the engine was just warming up or had just warmed up, and I was driving up a slight grade--1 or 2%

I looked up FSM about the causes to these codes, and it says bad gas could trigger these codes. I use 87 grade gas from Arco and another local gas station. Both are budget. Should I switch to a different brand and not budget?

Screenshot of the FSM, where it says bad gas could cause these codes:

View attachment 180870

I have also put in a bottle of Cataclean in the gas tank, drove till it was empty. Then filled up, after the fill up, on the same day, within the first 25 miles, the code came back!

I am planning on monitoring the Bank 1 O2 sensors voltages and fuel trims in the coming days.

I had the PO420 on a 2000 Odyssey, which was maybe five years old when the issue arose, but I never got the sister code - PO430. I got a Check Engine light when I was on a trip. I found I could reset the Check Engine light by disconnecting the battery. It would stay off for the rest of the day and perhaps for several days - I forget. I surely didn't want to buy a new catalytic converter. At that time, Odyssey cats were about $600, discounted on line, if I recall correctly.

I took the Scangauge II I had on of my other car and installed it on the Odyssey. These include an OBDII code reader that can reset codes while driving the vehicle, so I used the Scangauge to reset the Check Engine light as needed. Then I bought a second Scangauge so I had them on both vehicles. They will read most of what your engine computer reads, and other parameters like distance and fuel mileage..

I don't live in a state where I have emissions issues or inspections, but you may not choose to do this. It could be, your catalytic converter issue could clear up with time? If you do get a Scangauge II, it could buy some time to see if the issue clears itself.

 

@riverman, I have a Foxwell NT710. It can clear codes. I could also disconnect the battery, as you said.

In the past 30 days, it has come on twice. My plan is to monitor the O2 sensors’ voltages and fuel trims in the coming days. Will pay special attention to Bank 1 Sensor 2. If the voltage fluctuates, that’s usually a sign it is bad. Its voltage usually stays steady, contrary to the upstream one.

At the very least, I plan on replacing all 4 oxygen sensors. At 154 K miles, they are due.

I am doing a crap load of neglected maintenance by the PO.

 

I too replaced the O2 sensors at about 160-170k. The PO420 code had been coming on since about 90k, and I just put it out with the Scangauge whenever it tripped the Check Engine light. I replaced the two sensors on my 2000, not for the PO420 code, but because I was planning an extended summer trip, and is was preventative maintenance. I never got a misfire or other code to indicate the O2 sensors needed replacement. It's a guess, but I don't think replacing the O2 sensors is going to fix the catalytic converter code. If I lived in cold weather, I'd wait for spring to replace the sensors? Have fun!

 

In my case (2012 with 124k miles) the car was actually bad. Broken ceramic inside the cat. No oil, just white dust and white broken pieces. Of course I’ve tried all cataclean, Techron, etc and no results. I don’t like acetone or lacquer thinner as it can screw seals and orings on fuel system.

Bank 1 cat (Walker) from rock auto and upstream and downstream sensors from Autozone (Denso). Feels like a new car.

(Vcm was disabled when I bought the car at 120k miles, 5 months ago)

 

In my case (2012 with 124k miles) the car was actually bad. Broken ceramic inside the cat. No oil, just white dust and white broken pieces. Of course I’ve tried all cataclean, Techron, etc and no results. I don’t like acetone or lacquer thinner as it can screw seals and orings on fuel system.

Bank 1 cat (Walker) from rock auto and upstream and downstream sensors from Autozone (Denso). Feels like a new car.

(Vcm was disabled when I bought the car at 120k miles, 5 months ago)

I'm curious how that happened (the bolded part)?

Did something hit the cat from under?

 

(cough cough)... Non foulers... (cough cough)

https://www.odyclub.com/threads/2013-exl-with-146k-miles-p0420-and-get-shaking-vibrations-sometimes-during-highway-driving.376065/#post-2573770

 

Reading through all of these threads and trying to interpret each one’s comments/cases…

I own a 2018 odyssey EX-L with o/a 121.5k miles now. Tonight while driving, my check engine light came on stating I had an emissions system issue. I own a “OBD2” scanner paired with the Repair2Sol app, and did an engine system scan. The results turned up the P0420 - CATALYST SYSTEM EFFICIENCY BELOW THRESHOLD B1 code (also stating that it is a PDTC and cannot be erased by a scan tool). All that needless to say, what is the likelihood that I am needing to fully replace the Catalytic Converter altogether??? Could it be just some O2 sensors?

 

I don't mean to be a wise guy, but have you read this whole thread? That's what we are discussing.

We can't sit here from 500 miles away and tell you what it is, or even give you a percentage of likely (although I always tilt towards the Cat). The fact is that this code has multiple possible causes. Could be sensors, could be cat, could be exhaust leak, could be something else.