on 12/3/2021 - our 2012 Honda Accord 4door Sedan V6 engine with 132,970 got a P0420Catalyst System Efficiency Below Threshold (Bank 1) check engine light that came on - we used I believe 1 can of seafoam initially when the tank was probably half full and that actually cleared the check engine light. Upon the next required fill up we waited until it was pretty low on gas and did a 2nd can of seafoam just as an extra measure and drove it around for 10 miles or so and then filled up and it has been fine ever since that point.

We have noticed no sluggishness or poor fuel consumption ever on this car.

Now, today on 1/13/2022 it has 134313 miles the car was just filled up earlier in the day to full, and then later on the same day the check engine light has come on solid again, and the code is P0430 Catalyst System Efficiency Below Threshold (Bank 2) (instead of the original Bank 1).

We were told the car is a bit young to be having a failed catalytic converter the first time this occurred and were told to just add the regular seafoam and monitor things.... and there are no apparent symptoms we can see/feel..

So do we just keep feeding it Seafoam? Everything we read makes it seem like repair places will just rip and replace the converter.... is this common for this car?

my diagnostic thing i have on the car says it has been driven 1,343 miles since the last time this occurred, 57 gallons of gas and supposedly getting 23.45MPG (not sure I trust that MPG result - but just mentioning)

Any thoughts about this from experience?

 

I added a 12oz. bottle of Marvel Ultimate Fuel and Motor treatment (it says on bottle to use 12oz for 15 gallons of gas I believe) but online it says to use 6oz. typically for a passenger car...so wasn't sure.. but unfortunately I had a full tank of gas whereas I'm sure it would have been better to half a 1/4 tank full or less and then put in... but I didn't have that option so I just dumped the whole bottle in... drove it for about 20 minutes on highway speeds.. waited a bit, drove again on mainly highway for 20 minutes and that entire time the Check Engine Light was still lit... however on my way back I started up the car and the Check Engine Light was off..

So it cleared my check engine light - and my diagnostics thing is no longer registering anything. I'd say it worked at least for the moment. Repair place (Honda) said it isn't dangerous to drive with and is common for these codes to trigger and it could be running the tank a bit low of gas and less than premium gas causing it to trigger. Especially since no sluggish acceleration or lack of power type symptoms. They were pretty busy today so they also suggested to use Techron over the Seafoam, but Techron directions were pretty clear to not use when tank is full... so I thought I'd try the Marvel this time and it worked faster than the Seafoam I had used the first time we got a code.

So there ya go folks! Will update if the problem continues! Any thoughts are still appreciated!

 

P0420 means the O2 sensor in the converter is detecting the catalyst in the converter is not functioning above an acceptable. Since 1996, the federal government required the secondary O2 sensors as part of the OBD2 protocol. Early on, the P0420 could be caused by a faulty secondary O2 sensor in the converter. I'd say by the early-mid 2000s the P0420 is more likely a converter issue vs an O2 sensor issue. An O2 sensor is much cheaper and easier to replace than a converter (DIY part price is ~$100 for a sensor vs ~$1000 for the converter depending on your model accord).

The fuel additive may be a temporary fix. The question you need to address is why your bank 1 converter (toward the cabin/firewall) is degrading. Spending the money and installing a new converter will fix the problem, but you can degrade the converter over time if your engine is causing the converter to fail prematurely.

Since you have a V6 and sounds like you have a scanner, you can monitor the long and short term fuel trims on bank 1 and bank 2 when the engine is idling and warmed up. For each bank, the sum of the long term and short term should be zero if the engine is running perfectly. Any total fuel trim outside of the -10 to +10 range means you are running lean or rich on one of banks. Bank 1 is near the firewall/cabin and bank 2 is closer to the front bumper on your engine.

For starters, monitor the fuel trims on both banks 1 and 2 at idle, then at 2000 rpm. Let us know what values you get. If bank 1 is much different than bank 2, then you will have a starting point to troubleshoot any problems.

Also, have you done or had any work done on your accord not long before you started getting a P0420?

 

Thanks for the great reply! Your point about the engine degrading the converter over time causing the catalytic converter to fail prematurely. My question would be, what is my engine doing that is causing the catalytic converter to fail prematurely and if I replace the converter, why wouldn't I think it won't happen again if I've not addressed what the engine was doing to have the converter fail early?

Since my diagnostics thing is real-time like a tracker type device where the app pulls information from the engine and reports it via their little phone app, I'm not sure if it would actually actively report to me if the 'Fuel Trims' on either banks are at -10 or + 10 ranges? If possible, it hasn't done so yet.. but I suspect my diagnostic thing isn't like the normal ones where you can connect it and setup the test like you're recommending and monitor from the diagnostic app the fuel trim readings when doing 2000rpm vs idling. I suspect the honda dealer could do it, but they'll just recommend rip and replace I'd guess to get ya out of there and make money on the visit. So far though after the Marvel, it cleared up the 2nd bank check engine light and nothing further yet.. but I would be curious on your response to the question above. Maybe I'll pickup a cheap separate OBDII type monitor thing to do as you suggest. (good to have anyways, too bad the little tracker thing doesn't behave like a normal one vs. it just feeding you what they want you to see and report about..)



No work has been done on the vehicle aside from oil changes etc..

As well, upon the next low gas event, we did add the bottle of Techron....just because. It's getting low fuel again, so might be time for the ol' CEL to come on...will see...

 

Just try reading the fuel trims on both banks with your scanner to see what you get.

 

Does your car burn oil?

Many 8th generation Accords (both 4 and 6 cylinders) had excessive oil consumption.

Consequently, oil contamination of the converter may cause the converter failure.

 

I will pick up a cheap OBD-II monitor from somewhere and see what it results. My current thing is more like a GPS tracker connected to the Diagnostics port in real-time and it reports problems that it is checking for and that are thrown by the engine and its various sensors. It is pretty handy, but not as much in this case to do a deeper dive on the diagnostic portion... which is unfortunate, because there really is no reason for them to not have made it like all the other OBD-II diagnostic products out there.. but I digress...

also - I don't know if this car is burning oil, but the other poster that discussed that, thank you for pointing out that is the type of thing that this car could have problems with and that could be what is causing the catalytic failure in the first place. Something to be aware of!

 

Those V-6's with eco mode burns oil past the o-rings. I replaced all three cat's from a customers car and also add the VCM bypass. Chevy's from the same years had worse issues. Seems everyone did the eco mode ahead of it's time.

 

Thanks for that post with the pictures. Since I don't work on the cars myself really I'm not entirely sure what I'm looking at, but I presume that all 3 pictures are filters of some sort tied to the catalytic converter and where I see deposit buildup, it should in reality be clearer... and I presume when I'm using Techron, Seafoam or Marvel it is 'clearing' this up temporarily causing better O2/fuel mixture and not being a burden on the expensive catalytic converter - in layman's terms...? ....but if my car is burning oil, it could be what is causing those deposits to form when they shouldn't be.

I did take the car to get an oil change and they checked on this issue and said they saw no other codes and that all looked fine - and we will just keep monitoring if it continues.... granted, I don't know that they checked actually anything like what you're showing above or not or if they just saw there was no diagnostic codes present.
Also - I did get another OBD II Bluetooth BAFX diagnostic sensor $25 and Torque Pro android app for 5 bucks. Pretty cool little app and there are many other available free apps to download that can also connect to that. It does check the O2 sensors and all sorts of stuff and it all checks out as if there is no problem. I will have to fiddle with the Fuel Trim stuff (that is available to monitor in real time) that the other poster had mentioned above to see what those look like to get a baseline of what that looks like 'before' the next CEL comes on...

 

Those pictures are looking into each catalytic converter on a V6. The mesh/screen material is ceramic with small amounts of rare metals like platinum, rhodium, and palladium that do the chemistry on the engine exhaust. That mesh pattern gives a large surface area to the exhaust to react and meet emissions.