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Old 03-21-2011, 02:39 PM   #1 (permalink)
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Default Catalytic Converter Citric Acid Bath P0420

I just wanted to report success with a Citric Acid bath for my Cat ('01 2.5 NA gas). If you Google Citric Acid Catalytic Converter, you'll see what I'm talking about. Sorry, I can't post links due to being a new poster.

I too had the frustrating P0420 code, yet the emissions testing facility said my exhaust was a PASS (with flying colours too).

I tried replacing the rear O2 sensor, but the code would still come up. I tried the $5 mechanical fix, but I didn't have the spacing right, so the code still appeared. Plus I wasn't comfortable just ignoring the fact that the code was being generated.

Last year (April-May), I did a whole ton of work when the driver's head gasket started weeping coolant. While I had the engine out, I decided to try a citric acid bath for the catalytic converter. I made up a solution (can't remember the dilution exactly right now, but it was in keeping with that study on restoring cat efficiency for diesel cats - yes, diesel cats - ours is gas, but I tried it anyways). I fired up the BBQ and got a honking large pot (from the dollar store/Bargain Shop - cheap $15 stainless pot made, poorly, in India).

I kept the solution at around 70-80 deg C (165F) and let it simmer for 6 or 7 HOURS, periodically swooshing it around. The solution became a gunky brown/grey. And the cat came out all nice and clean-looking. I rinsed it all profusely with fresh water.

I put the car back together. I also replaced BOTH O2 sensors - again, while I had everything apart. The car had 220,000km (140mi) on it, so I just went a little crazy and replaced all kinds of things while I had it all apart.

Anyways, I have NOT had a P0420 since, and it's been almost a full year. So I can (at least somewhat) attest to the citric acid bath being effective. I certainly don't think it will harm the cat, if you do it "right". I did consult with a Subaru tech about just replacing the cat. His position was that the code is there for a reason, and the mechanical "fix" was a joke. He also advised against aftermarket cats, as his experience was that they are very problematic. So it was a choice between buying a new OEM cat (big $$$), or just ignoring a "valid" code. That's why I gambled on the Acid bath.

So far it's working wonderfully! I just thought I would report my findings to the group.
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Old 03-21-2011, 05:16 PM   #2 (permalink)
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Wow, I haven't heard about this.

Scientists at Madrid's Institute of Catalysis and Petrochemistry have discovered that diluted citric acid can remove these deposits without affecting your converter's platinum. Just soak the catalytic converter in a citric acid solution for six hours; and if it doesn't have other problems, it'll be as good as new.

Ways to Fix a Catalytic Converter | eHow.com

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Old 03-21-2011, 05:54 PM   #3 (permalink)
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Where did you get the citric acid? Did you buy water softener cleaner?
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Old 03-22-2011, 08:49 AM   #4 (permalink)
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I got the citric acid powder from a farm supply store nearby. But I needed more (it's a great cleaner for all kinds of stuff, and very eco-friendly), so I ordered it from a local brew-it-yourself winemaking store. THey had little packets for like $5, but I ordered a kilogram (2.2lbs) for around $10. I used maybe half of the kg for the cat, so about $5.

IIRC, the powder was diluted in water using a "formula" of 100grams in 1 litre makes a 10% solution. Or 1gram in 100ml makes 1%. Or maybe you are supposed to count the water as part of the mix - so it's technically 1g of powder to 99g of water for a 1% solution. Sorry, I didn't get very far with high school science.

The Cypriot research (if you google "Reactivation of an Aged Commercial
Three-Way Catalyst by Oxalic and
Citric Acid Washing", one of the links gives you the article for free) also used OXALIC acid. They compared the two different results. I think they said oxalic was BETTER for certain contaminants. But that citric alone was almost as good, so I just used that. Again, the research was for diesels. They used a 0.1M solution, and I don't speak "chemist talk", so I don't remember how to convert that to %. Maybe someone smarter than me can explain.

I think I used around a 10% solution? I know I use a 5% solution to clean the toilets, trying to keep it pretty mild. So it must have been around 10-15%? Nuts, I wish I remembered the exact details. If you google citric acid in relation to de-scaling kettles or boilers, there is tons of advice. I think lemon juice is around 5%. Or figure out what 0.1Mol means.

Sorry, I should have written all this down. I'm the type that figures out the answer for the question at hand, and then promptly forgets most of the details. Plus it's been almost a year.

I didn't use any special water - except to say that I have multiple filters (carbon & coconut shell based, NOT RO) on the tap water, and our water is not very hard, and doesn't have lots of iron, etc. I suppose the best water would probably be distilled. I think RO is too "hungry" because it's been stripped of pretty much everything, making it unbalanced. And that's why you're not supposed to drink it. But this is another can of worms, and the Coca Cola company would probably disagree.

Sorry, I don't know what water softener cleaner is. Is it some kind of solution that you are supposed to use to periodically de-scale your water softener? We don't have one, so I'm just guessing.

Hope this helps.
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Old 03-22-2011, 08:52 AM   #5 (permalink)
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Oh, and I just read the ehow article. It says "just soak it for 6 hours...", but I'm pretty sure the heat makes a huge difference for the effectiveness of the acid. The ehow is a little over-simplified.
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Old 09-24-2011, 02:03 PM   #6 (permalink)
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Update for you all:

7 months later, and still no 0420. Running clean and smooth. That brings it to 18 months now since I did the bath. The car now has 256,000km on the original cat, over the course of 11 years.
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Old 09-24-2011, 02:04 PM   #7 (permalink)
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Has anyone else tried this? I'd like to hear their stories...
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Old 10-24-2011, 08:39 PM   #8 (permalink)
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Interested in more stories, too. I assume you had to remove the cat completely to do the clean?
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Old 11-26-2011, 05:17 PM   #9 (permalink)
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I'll be trying this next weekend. I've got everything shipping today from Amazon (gaskets, bolt hardware and acid). I'm not sure if this will work due to the fact that I think my cat is rattling, which may mean its just damaged. Only one way to find out.
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Old 11-30-2011, 01:59 PM   #10 (permalink)
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Default Lemon Juice

I have a 2004 Legacy GT with about 137k miles. I was just at the Dealer today trying my last option to get rid of the P0420 code. I attempted to have a newer version of firmware flashed on to my ECM. However, no luck since there was not a newer version.

Prior to this I've tried replacing the rear 02 sensor and also recently changed my PCV valve. Also checked for a vacuum and exhaust leak. Found nothing and still have the annoying P0420 code.

Like everyone else, I'm trying to save from having to buy a new CAT (~$2000 for the whole cat setup from the dealer). Before I left the dealer today, they gave me one last recommendation. Try out Lemon Juice. Apparently they've been using that method on atleast 10 vehicles and all but maybe 1 has worked and for a year plus.

This weekend as well I plan to take off my cats and just use store bought lemon juice to flood my cats with and leave them sitting for 1+ days. I already bought the replacement parts, I'm just waiting for the weekend to roll around. I will try to post back after the job is complete with my results. Good luck everyone!
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Old 12-03-2011, 07:25 AM   #11 (permalink)
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I tried this two days ago and so far, I'm still without my P0420. I wrote my own how-to that differs from Zenon's in a few key areas, but still linked to this post for proper credit (thanks again, Zenon, I'm really hoping this works). It's here.

I'm taking the car in for an emission test either today or tomorrow. I'll write back how it went. I know the car has to do a whole drive cycle after the codes have been cleared (I wasn't aware of this). A post describing the forester drive cycle is here.

Good luck to anyone who tries this.

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Old 12-05-2011, 03:21 PM   #12 (permalink)
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Default Worked for me!

Past emissions today with "flying colors" (tech's words, not mine). So, I'm pretty stoked about this. All-in-all, only cost me two gaskets, an O2 sensor, and five pounds of citric acid.

I'm going on five days with no CEL. Previously, if I ever reset the codes P0420 would be back in an hour or two.

Thanks Zenon!
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Old 12-05-2011, 06:05 PM   #13 (permalink)
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I wonder if this wouldn't work for me, too. I think I have pretty much narrowed my P0420 to bad cat or a small leak some where before the cat. O2 sensors have all been replaced and the car is running great aside from the CEL for the cat code.
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Old 12-24-2011, 10:55 AM   #14 (permalink)
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Greetings, I too am paying attention here.... mine is an '04 Outback NA, PZEV model with 3 cats and same long term P0420 problem. Thinking it's my next try, I sure don't want to plunk down $2100 for new cats if if I can get away with not....
Bought gaskets thinking about giving it a try.

What's the latest report from revvy and Zenon?

My background on the rig if anybody is interested:

2004 PZEV w/420 code will Subaru help w/cost to fix? - Subaru Outback - Subaru Outback Forums

Merry Christmas...
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Old 01-06-2012, 06:43 AM   #15 (permalink)
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Going to give it a go here soon... was wondering if Zenon or revvy had any more advise on ratio of citric acid powder/water to use. I've got 20 lbs. coming from Ebay to play with, sounds like big plenty.

My setup(2004 Outback PZEV) is two cats in one pipe assembly(driver side) and the third one in the other side pipe. That makes the driver side assembly about 4-1/2 feet long, thinking that will take quite a few gallons to submerge it. Haven't got a vat figured out yet, I'm thinking about my tractor front loader bucket.

How about Soob4Life, any luck?
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