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California98Civic 06-16-2017 02:32 AM

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:


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.

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:

[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:

Here is another guy who had success with a similar method:

Daox 06-16-2017 10:00 AM

Awesome info! Thanks for sharing.

Have you gotten test results after doing the cleaning?

Stubby79 06-16-2017 11:56 AM

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

oil pan 4 06-16-2017 12:48 PM

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.

ChopStix 06-16-2017 01:00 PM

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.)

California98Civic 06-16-2017 04:36 PM

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! ��

oil pan 4 06-16-2017 07:17 PM

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.

California98Civic 06-17-2017 10:53 AM


Originally Posted by oil pan 4 (Post 543066)
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!

gone-ot 06-17-2017 12:26 PM

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

California98Civic 06-17-2017 01:46 PM


Originally Posted by Old Tele man (Post 543128)
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.


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