Restoring an old catalytic converter's efficiency (cleaning method for P0420 code)
 

EDIT: This post has been edited to update technique to how it went second time doing it. The successful test results are posted on p.6 of this thread, here: https://ecomodder.com/forum/showthre...tml#post573356

Introduction:

This post reports my procedure for an at-home “acid washing” of my car’s old catalytic converter. Cleaning can save hundreds of dollars compared to buying a new three-way catalytic converter ("CAT"). The method was published in Environmental Science and Technology in 2006[1].

Why clean?
For several years, in my smog tests, I have seen the emissions get worse on my 1998 OEM CAT that still serves my Civic DX. The car still passes, but only barely in one category. Some readings for HC, CO, and NO are 4-6 times the average for this car. This spring (2018) I got the check engine light P0420 code persistently when the engine developed a head gasket leak and spark problems. This cleaning procedure has been done twice now. The below reports the 2016 process with added info from 2018 process.

Cleaning is almost as good as buying new:
A solution of over the counter citric acid and oxalic acid in pure water strips contaminant off the catalyst without removing the catalyst itself. The result can be to restore 90-95% of the original function of the catalytic converter.

Visual Inspection:
In 2016, after removing my catalytic converter (“CAT”), visual inspection of it before cleaning showed lots of white, fine, flaky powder on both top and bottom of the substrate inside the housing. And there was so much of it that it seemed to more or less plug the converter in many places. Some of it was loose enough that it fell out through the O2 sensor holes like little flecks and clumps of powdered sugar. Most of it was stuck to the honeycomb and both oxygen sensors. But the honeycomb still showed a cross-hatched grid pattern without any dark crud or structural crumbling visible. In 2018, it still looked quite clean, but of course the P0420 code proved otherwise.

Products I bought, borrowed, or already owned:
Anthony’s Citric Acid powder (5lbs): $18.
Cheapo 12 Quart pot: $9.
Savogram Wood Bleach (340ml @ 95-100% Oxalic Acid): $8.
Bottled water (5gal): $7.
Propane use: maybe two dollars-worth.
Candle-making thermometer: borrowed for free.

The formula & procedure:
Caution: do this outside in a well ventilated area and wear gloves and goggles. The acid is not particularly caustic, but you don’t want to get it on stuff unnecessarily. Also, while heating it produces a smell that would not be pleasant inside—and that might indicate something unhealthy.

Formula: Per gallon of water I used 75ml citric acid and 50ml Oxalic acid.

Procedure: I placed the CAT in my cheapo 12 quart pot. Two and a half gallons water nearly reached the brim and completely covered the catalytic converter inside the housing. So I mixed 190ml Citric Acid and 125ml Oxalic Acid with the 2.5 gallons water. Ideal temps for the solution are said to be 150-160* F for six hours. In 2016 the tempertures were higher than recommended (185 or 190) most of the time. In 2018, I got it just right. This second time I lifted and submerged the CAT in/out of solution six times each half hour to ensure flow through the substrate.

Results:
In 2016, all the chalky white deposits were gone, top and bottom of the substrate. What was left was the clean slightly darker, grayish honeycomb—looking perfect but wet. In 2018 there were some darker gray spots. No damage. The $45 or so that I spent is much cheaper than even the cheapo eBay CAT replacements. Results of my smog tests are here: https://ecomodder.com/forum/showthre...tml#post573356

Endnotes:
[1] “Reactivation of an Aged Commercial Three-Way Catalyst by Oxalic and Citric Acid Washing” by S.Y. Christou, H. Birgersson, J.L.G. Fierro, A.M. Efstathiou, Environmental Science and Technology. 40 (2006) 2030-2036. The abstract for this article states: “The efficiency of dilute oxalic and citric acid solutions on improving the oxygen storage capacity (OSC) and catalytic activity of a severely aged (83 000 km) commercial three-way catalyst (TWC) has been investigated. Washing procedures applied after optimization of experimental parameters, namely, temperature, flow-rate, and concentration of acid solution, led to significant improvements of OSC and catalytic activity (based on dynamometer test measurements) of the aged TWC. The latter was made possible due to the removal of significant amounts of various contaminants accumulated on the catalyst surface (e.g., P, S, Pb, Ca, Zn, Si, Fe, Cu, and Ni) during driving conditions, as revealed by Inductively Coupled Plasma-Atomic Emission Spectrometry (ICP-AES) and X-ray Photoelectron Spectroscopy (XPS) analyses. For the first time, it is demonstrated that dilute oxalic acid solution significantly improves the catalytic activity of an aged commercial TWC toward CO, CxHy, and NOx conversions under real exhaust gas conditions (dynamometer tests) by two to eight times in the 250−450 °C range and the OSC quantity by up to 50%. Oxalic acid appears to be more efficient than citric acid in removing specifically P- and S-containing compounds from the catalyst surface, whereas citric acid in removing Pb- and Zn-containing compounds, thus uncovering surface active catalytic sites.”

Here is the video discussion of the formula that I adapted to my materials: https://www.youtube.com/watch?v=64KwPL0PHoY

Here is another guy who had success with a similar method:
https://www.youtube.com/watch?v=Kv4PxJKtKxU

Awesome info! Thanks for sharing.

Have you gotten test results after doing the cleaning?

I like it! I just hope I can find said chemicals when it's my turn.

The chemicals are very easy to get the oxalic acid is not too hard to find, citric acid is very easy to find.
I'm cheap so I just use air conditioner condensation when I need distilled water.

Also mining new platinum, palladium and rhodium for converters is incredibly environmentally destructive.

Palladium is at like a 16 or 17 year high right now, but rhodium and platinum are relatively low.
It was low enough to where I bought some rhodium, last year because it has gone up to $13,000 per ounce in the recent past.

Another important point, proper disposal of the used chemicals.
There is a very high likelihood that some of the ash in the converter is lead or cadmium bearing. So dump it down the drain. Waste water knows how to prevent the release of lead back into the environment since some houses still have lead or lead jointed sewer pipes.

I have one question related to cleaning a converter. There was a News story a while back talking about weird guys who have been vacuum sweeping the sides of highways. Because they claim that Cats discharge the precious metals which collect in the road side gravel. And thus they claim that there's money to be made from recycling the metals separated from the road side gravel. So correct me where I'm wrong. But it seems that once the metals are gone that the converter would become much less efficient, and thus I'm not sure its worth cleaning one when its life span has been notably reduced from use. Am I wrong about this?

(You can search about collecting the road side platinum... I'm not linking to it.)

I'll try to answer each of these comments quickly. First, I have not been able to retest. My troubles with the smog testers rendered it more difficult and delayed me long enough that I wondered if it would be valid anymore. The chemicals are easy to find. I bought what I used online. Look up the brands for the chemicals in my list and you'll find them. On the question, I can say that I disposed of them down the toilet. I totally agree about the toxicity of the mining, which was also one of the things that I was imagining would be a benefit of doing this. On the idea that maybe the catalytic converters fall apart as they get older and drop their precious metals onto the highway: I'm not sure I believe that. The authors of the scientific studies seem to feel that was perfectly normal to expect that an aged converter would still have its press metals intact. That said, mine is just one car. If my car lost 0.5 percent of its precious metals over the course of 20 years, it would make very little difference in the operation of my catalytic converter I would imagine. But if each of several millions of motor vehicles crossing along a single stretch of freeway multiple times per week lost 0.5 percent of the precious metals in their catalytic converter over the course of a group of years, then maybe some significant amount of these metals would accumulate in the soils on the margins of the freeways. Still seems a little bonkers though! ��

I have seen converters come loose and shake around in side the housing, but it makes a lot of rattling.

As an engine gets older it starts to send more oil through the exhaust due to worn rings and old or worn valve stem seals.
The detergent and anti ware additives will ash up the converter.
Cleaning will have to be more done more frequently until the engine is rebuilt.

Yeah. That kind of failure. But it will be relatively rare. That's not the life cycle of most CATs!

Your next CARB-emissions test numbers will "tell the tail" of whether it was worth it or not.

My plan had been an A/B test, but I ran into trouble with testers and the CARB people in Sacramento on unrelated technicalities... So I got delayed. Then after passing, then cleaning, I was back on the sniffer machine paying for a second test and was told I had oxygen infiltration .... So no test. By the time I fixed my muffler (mee self, custom), it was five months on and I was no longer confident the test would be really valid. This old engine probably burns a little oil and maybe leaks a little fuel too. So I would expect several thousand miles to change results a little. But TRUE, my next smog test will be interesting, anyway. I have decided to wait for it.

BUT REMEMBER, THERE IS NOT CONTROVERSY AS TO WHETHER THE PROCESS WORKS AS DEMONSTRATED IN THE SCIENTIFIC PAPER I CITED. THE ONLY QUESTION IS WHETHER AN ADAPTATION OF THE METHOD WITH EVERYDAY TOOLS CAN WORK. ��

This cleaning process works.
 

I have used it several times in the past with success. I mentioned this in other threads on the topic.

The most dramatic was a 2007 Dodge Caliber 2.4 L with a very expensive "Mani-verter" that had been coked up when a failed upstream O2 sensor and a ditzy wife conspired to leave a cracked and smokey exhaust system just a month from the strict bi-annual emissions test after she drove around for weeks with an engine light on. I TIG welded the cracks in the stainless housing and followed the mild acid wash process with the only change being an initial washout with "Purple Cleaner". This caustic cleaner removed the majority of the carbon coking. The mild acid mix concentration should be closely followed as a stronger acid reaction will strip away some of your precious metal loading in the converter.

The vehicle has an upstream and downstream O2 sensor and an algorithm to check converter efficiency by comparing the two sensor outputs. It also has test cycles the CPU runs periodically by pulsing the fuel mix rich and lean in a coded sequence with expected reactions from the O2 sensor compared to the actual. This prevents a simple O2 sensor defeat device on the downstream side allowing such shenanigans as cored out cats or straight pipes.

The factory mani-verter was eventually replaced with an improved aftermarket unit, but the cleaned converter did run for several years after the cleaning process.

Thanks for your experience. Yeah, I have seen a few testimonials. My test was supposed to give us readings A/B, direct from CARB that could show us how effective it is (or isn't). But as I explained above, the CARB gods were not smiling on me. So no test. I posted the process I used for others to consider. Careful critical reading is always expected/welcome. Thanks!

Hello California98Civic,
Huh, the paper you linked is interesting - seems like a workable idea. The whole thing is available on ResearchGate as a pdf. Bummer on the no re-test. Did you happen to keep your old smog results over the years before cleaning, to see the degradation over time? If so, you can still get some information by comparing the next few smog test results after the cleaning process to see if there's a change in the downward slope angle, or if the slope moved higher, indicating either it is working better or worse or degrading at a slower or faster rate. True, you won't have an immediate A/B test result, but you can look at the trend over time. More variables in play - state of the test equipment, what it's baseline setting was, etc. - but better than no result at all.

Poking around, I found an older thread by BamZipPow here: Catalytic converter cleaning would you do it?

He noted that he'd cleaned his 380,000 mile T100 converter and put 41,000 more miles on it after that - but I can't tell if he had any code differences before or after. BamZipPow, you reading this? Do you have any info on whether the cleaning was effective or not in your application, like a code going away/staying away longer or smog test results?

I'm looking for a way to clean an O2 sensor - it would seem that similar deposits would be to blame, and it is also a ceramic-based device, so maybe this method could also be used for them. Google is useless - the most cited method is to soak the whole thing in gasoline. Yeah, no. Second is to heat it with a torch to cherry red and dip in water to get the carbon to flake off. Seems to me that would shatter the ceramic. O2 sensors are not that expensive, though, so it may not be worth it - especially here in the rust belt where it might bust off trying to remove it in the first place. My guess is that my rear O2 sensor has bad wiring or is totally gunked over from the outside with oil sludge from the leaks over the years - it reads 0.0V all the time. Pain to get to, have to yank the center console and remove a panel to get to the top of the transmission from the inside of the truck to reach the plug.

BTW, there are a number of patents around for systems using the solution to clean catalysts, like this one for hooking up via the O2 sensor bungs to clean them in situ: US8888921

Interesting information. Thanks. I think you are correct that there may be some utility in the next test, even though there will have been a two year gap. But it just won't be as cool as A/B one day to the next. Oh, well. As far as "results" in the second vid I posted in the #1 post, at minute 7:39 the guy shares his evidence, which amounts to a code going away when he resets the computer and not returning. Not a bad data point for homebody garage job.

This is a timely subject for me as I now own a car with a catalytic converter. I chose to look into new prices:

https://www.carid.com/1979-volkswage...ic-converters/

They range from $48 to over $300. :confused: Any idea what makes the difference? For instance:
What I take away is that "Green Ultra High" is twice as expensive as "Race High Flow" though the descriptions are identical.

Kooks! Heh. I should take it to Goofy's Muffler Customs to have it installed (Kidding, I'll flush it first).

I didn't find it but thesamba had a post on replacing the activated charcoal in a charcoal canister (such as my Superbeetle has) using material from a $14 Cadillac part.

I read about the price differences a little when I was planning to do the cleaning. IIRC, I think a primary source of cost difference will likely be the amount of catalyst metals and therefore the long-term durability. Cutting back on the pricey metals would cut costs. In the short term both CATs will work fine, but the cheaper one will fail first. (Also, probably they save on labor in their supply chain through unsafe mines some sweatshop somewhere in the world.)

That's one reason I chose to clean (the OEM Honda CAT is 19 years old and still puffin' passable gasses!).

Oh... I forgot to reply to this part of cajunfj40's message:

Yes, I did save them. And yes, I will be very interested to see the change (if any) good or bad and post here!

I need to check out my cat. I read the best way to wash one was to throw it in the toilet, sit on the lid, and flush, but I am not sure how reliable "Dog" is. I drove to Mom's house and had a CEL when I was leaving her town. My bluetooth reader is in my Civic, which had been showing a code approximately once a month.

I never saw one in the Accord.

Finding somewhere open in sleepy small towns on Sunday took a while. The guy at O'Reilly said both of my catalytic converters were failing, but I failed to get the codes. I keep seeing P0420, but I have not been able to find out how many converters my car has.

I have heard of cats that sound like cans of marbles, ones that simply fall apart, and presumably ones that clog.

There are periodic nagging voices asking why they failed, the new one may fail also. Eric the Car Guy says Hondas and Toyotas do not like aftermarket cats.

This guy had a different method of cleaning it, which did not work: https://www.youtube.com/watch?v=L_QIRFUg8K0

Mining.com, Hackaday, and Gizmodo each have articles about mining highways, but Cody's Lab tried it: https://www.youtube.com/watch?v=v5GPWJPLcHg

People kept asking if he was going to keep sweeping the highway.

I asked if he was going to get a job driving a street sweeper.

Thanks. I didn't realize how much I needed to hear that again. :thumbup:

So the video proves we could mine... wherever the street cleaners have been dumping their catch daily for the last 40 years and derive precious metals. The dump? Futures of mining!

As for your accord and the cat troubles... I don't think a 2.3L inline 4 is likely to have two cats, do you? Pop the hood and look. The six cylinder was years later, I thought.

Toilets will not help you with this problem. Save them for their primary design purpose.

:)

james

The first V6 Accord was the 1995 model. I was finally able to pull up a parts diagram and it showed two different cats for the same part on the diagram, but I could not find a difference besides price, which is significanthttps://www.hondaautomotiveparts.com...S843B0200F.gif

One dealership only shows the KL cat for $1,256.95. I could purchase another Accord for that price! :) Majestic sells it for $461.45, plus a $150.00 core charge, $35.58 shipping, and $3.95 handling. Total: $650.98. I put in my VIN, so hopefully that is the right part. Do I need to pay for shipping to get the core charge back? It would be worth trying to find someone that would pay more for it, but I still need to see what is actually wrong with my my cat.

Jerry Seinfeld once had a bit about flushing a toilet and having the water flow up. That sounds preferable to my new place where the water simply flows around that which I am attempting to flush.

We need Al Bundy's toilet!

Now I am reminded again why I wanted to try and clean it. I wish the stars had aligned better for the AB test of my cleaning. But remember, it was scientifically PROVEN, which means it must work!

My replacement cat for my Jazz cost me £85.

Fine. But as has come up before, it likely is that cheap substantially because it has less of the catalyst, which is made of precious metals such as platinum. It will likely not last nearly as long as the OEM. But your mileage may vary...

Agreed. But as cats rust out here before they fail emissions, why worry.

Hey Mister! Have you considered going into the acid resale business? Do you have enough left to put into a suspicious package? Then I would just need to find a pot vendor and I would be in business! :D

Exhausts here rust from the outside in. Not from the inside out, as in most places. For most of the winter, with the amount of salt that's put down, and the constant rain, it is like driving in the sea!
We get frosty nights, so they salt heavily, then it rains during the days meaning they have to salt again the next evening. That continues for weeks on end.
Made worse here, as I live within 200 yards of the sea.

PM me. I have your acid. :)

Consider whether your posts reflect well on the forum, and don't attract un-necessary scrutiny.

Platinum broke $1,000ozt the other day and palladium isn't too far behind.
So I say clean those cats.
If those prices go much higher we are going to start having problems with converter theft again.

These are garden variety powders in tiny quantities that I received from a mainstream retailer in the mail. You make it sound like homeland security is coming to my door! :)

Absolutely correct.

Palladium is at like a 16 or 17 year high.
Platinum is coming off like a 5 year low of 900 an ounce. I bought a few ounces, so I'm watching the price.

I tried to find more information on this method.

3,000 miles before code: https://priuschat.com/threads/acid-b....166554/page-3
12,800 miles before code: https://www.tacomaworld.com/threads/...d-wash.241619/
18 months and no code: (All Years) Catalytic Converter Citric Acid Bath P0420 - Subaru Forester Owners Forum
Almost 30,000 miles without a code: P0420 / P0430 and a "Hail Mary Pass" (the "cat washing" thread) - Page 12 - Toyota Tundra Forums : Tundra Solutions Forum
BamZipPow ran it through the dishwasher and has not had a code for 41,000 miles: http://ecomodder.com/forum/showthrea...-do-23483.html
480 miles and disappered: https://www.bobistheoilguy.com/forum...topics/3391904
Tried a few things, without clear results: https://forum.ih8mud.com/threads/cat....833570/page-2
Attempted, but did not report back: (All Years) P0420 Catalytic Citric Acid Bath (How To) - Subaru Forester Owners Forum

Who knows how accurate any of this is, but there is indication that 50% of people who try this see clear benefits, but unfortunately, it seems like hardly anyone followed the directions perfectly.

Try hydrochloric.

This seems to indicate HCL would not provide the results I desire: Using Dilute HCl/Cl to Process Catalytic Converter - Gold Refining Forum.com

I think that Subaru link is one of the dudes I was copying in my citric acid bath cleaning. FWIW, thirteen months later and no emissions related codes (no codes at all, acutally). But I did not have a code to begin with. I had reason to suspect my 1998 OEM catalytic converter might fail emissions and wanted to be prepared. I passed emissions, barely. Then I cleaned it.

California98Civic was kind enough to share his cleaning supplies with me; I bought a 16 quart stainless steel stock pot for my larger Accord cat and put it on a hot plate for 10 hours. Unfortunately, the larger pot was still not deep enough, either I needed a much larger pot, much more water, and much more powder, or a turkey fryer pot. It did something, the water turned black. The water did not cover the electrolyte honeycomb, so I flipped it over halfway through, although since the sensors is in the middle I could have ignored the back half. Curiously, it ate one of my gaskets.
I did this at 179,270 miles.
http://ecomodder.com/forum/attachmen...1&d=1506362279

Hmmm, HCl might work a little too well.

This is great. I am glad there has been no CEL for nearly 2000 miles, too! My pot was also on the small side. But the honeycomb was completely submerged for me. And I used a ladel or something to move sort of baste the top of the honeycomb. Did you monitor solution temperature at all? It is not supposed to boil, but they recommend a pretty high temperature.

Unfortunately, I forgot to bring my infrared thermometer, but it was never boiling when I checked on it.

Did the acid eat the coating in your pot? Mine turned green, I cleaned it, and now it is rusted!

Cleaned O2 sensors with this method
 

Decided to clean my O2 sensors with this same method, rather than using gasoline on the sensors. I figured if the light acid wash was good enough for the catalytic converter, then it should be fine for its partners in crime (the O2 sensors). I used a half gallon of tap water instead of distilled. I cooked for two hours, scrubbing occasionally with an old tooth brush. Pics:

BEFORE (upstream on left, downstream on the right):
http://ecomodder.com/forum/attachmen...1&d=1526591967

The downstream one on the right was throwing the P0420 code.

Submerged the contact area of the sensor only:
http://ecomodder.com/forum/attachmen...1&d=1526591999

After cooking (no boiling) outside for 2 hrs, compare the above and below photo to notice the flakes of yellow and black in middle of the pot that have gathered there due to stirring:
http://ecomodder.com/forum/attachmen...1&d=1526592026

AFTER (upstream on left, downstream on the right):
http://ecomodder.com/forum/attachmen...1&d=1526592049

Took it for a spin of maybe 15-20 miles. All seems good, Felt like I got into DFCO easier, sooner, but that could easily be confirmation bias. Butt dyno stuff, that.