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Old 06-16-2017, 01:32 AM   #1 (permalink)
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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:


See my car's mod & maintenance thread and my electric bicycle's thread for ongoing projects. I will rebuild Black and Green over decades as parts die, until it becomes a different car of roughly the same shape and color. My minimum fuel economy goal is 55 mpg while averaging posted speed limits. I generally top 60 mpg. See also my Honda manual transmission specs thread.

Last edited by California98Civic; 07-06-2018 at 08:35 PM.. Reason: Update
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