Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Catalytic Converter

I have a 2002 subaru forester with the check engine light on for the catalytic converter. I know I am going to replace this part but i am getting mixed opinions about how I should do this. One mechanic swears that I need the factory part, a cost of over 1000k. Others laugh at this and have offered all sorts of alternatives including cheaper, after market parts. Is there any solid technical evidence to justify the cost of the factory replacement part? Is my car going to run poorly if I jerry rig the converter or buy an after market replacement part?

An aftermarket catalytic converter will work just fine. Before any aftermarket catalytic converter can be sold it must first be certified by the EPA that it functions the same as an OEM catalytic converter.


As long as it’s a “direct fit OEM replacement” aftermarket converter it’ll work fine. The problems occur when people (and some chain muffler shops) try to weld in a “fits all” generic converter.

Are you sure it needs a new converter? I have a feeling that a lot of shops are making good money these days replacing good cats with new cats. That CEL that points to the exhaust, may well be something other than the converter. Don’t assume every time the light comes on it means to replace the converter.

The part that made the CEL come on is the rear oxygen sensor, not the converter. I would replace that first…If you DO replace the converter, replace BOTH sensors, front and rear, to protect your “investment”…And yes, forget the dealer…

There is a man, Scott Kilmer, with a Youtube video who says he adds a gallon of lacquer thinner, not paint thinner, to the gas tank when it’s half full, which makes for a 10% solution, and it cleans the converter. He says he has been doing it for many years and most of the time it fixes it. He said you get a fail when the efficiency drops to only 95%. He says such things as run the car parked in your drive at 2500 rpm for an hour or so, then drive it out on the highway for at least 60 miles with the thinner in it.

He warns this only works on contaminated, clogged converters, not on damaged ones, though I think that would be obvious.

Mine failed on a recent trip down the mountains into Orizaba, a long; long drop running 3500 rpm in second gear. We stopped at a restroom, and as I drove on the CEL came on. My scanner said the dreaded P0420 code.

It takes two failures to give the CEL light. We drove on after I erased it, and when we got to our destination, it was pending again. On the way home, yes, it failed.

I would be willing to try lacquer thinner if I could find it. I thought I knew how to say lacquer in Spanish, but when we went to a Comex paint shop they had no idea what I was talking about.

Kilmer also showed taking off the converter which can be really hard on a rusted old model, and showed tossing it overnight in a bucket of water with dish soap in it.

I did a lot of Googling, and a lot of people had theories why it couldn’t work, and why it would destroy your car. The funniest one was the guy who said lacquer thinner is flammable and would probably burn up your car. Like gasoline is a flame retardant, I guess?

There were also people on various forums who said they had tried it and it worked.

As indicated, that assumes the converter is really the problem. He showed in his video the crud built up in the fine rare metal grid inside the converter, then it was gone.

One thing for sure, if it wrecks the already failed converter, it can’t be any more wrecked than it already probably is.

There was a question about clogging a fuel filter by knocking crud out of the gas tank. It is only a 10% solution as he recommends it.

Also, questions about plastic parts in the engine where gasoline goes, being harmed by it. He claims not, but I imagine modern gasoline is pretty nasty stuff, already.

I cannot recommend it, because I haven’t tried it. My wife goes back by bus, and I may ask her to bring 2 - 1 quart containers of lacquer thinner with her, if I can’t find the correct stuff here.

Is it possible? I’d have to say it’s possible. Just not sure.

If I were in the US, I’d take it to the dealer and simply pay what it costs. With 175,000 miles, I don’t begrudge this wonderful car a repair once in a while, though so far it hasn’t needed too many.

lacquer thinner is mostly acetone, and it will NOT clean your converter because it will never reach your converter. It will burn completely in the engine. Acetone can indeed damage the plastic and rubber parts of your fuel system…It’s octane rating is unknown but I suspect it’s very low which will invite severe detonation…Good Luck…

I understand what you are saying. In fact, most people who never tried it, and I googled extensively, there was discussion on my boards, all said the same thing.

The very few who did try it reported it worked. I commented on another thread on this issue and will be doing some deep study into this.

Some of the fuel makes it to the exhaust unburned or partially burned.
That’s one of the reasons the converter is there in the first place.
So it’s conceivable that lacquer thinner could do…something.

The very few who did try it reported it worked. I commented on another thread on this issue and will be doing some deep study into this.

There are people who swear that using Slick-50 is saving them hundreds if not THOUSANDS of dollars a year in gas. But test after test after test has proven that it doesn’t…never has…never could. There have even been tests run on peoples cars who claim that they’re getting much improved gas mileage…yet the tests concluded that their mileage is well within spec. But the people don’t believe it. People grasp onto these glimmer of hopes for improved gas mileage (even though the it’s a false hope).

Sorry c-smith, but in a fuel injected, warmed up engine, NOTHING makes it into the converter in liquid form. When fuel is injected into a warm engine, it INSTANTLY turns into a gas and after it burns, there is NOTHING left except combustion gasses…

When the catalyst in a converter fails, NOTHING you can pour in the gas tank will restore it’s function…NOTHING will “clean off” or remove combustion deposits which take on a ceramic form…These converter killing deposits are the result of impurities in the gasoline or metallic additives in the motor oil which slowly kill the converter…

Caddyman, you seem unable to comprehend what I said.
I said NOTHING about any liquid.
I NEVER said lacquer thinner would work in a helpful way.

So, go find somebody else to browbeat.

Good day.

Caddyman and Circuitsmith have both made good points. Caddyman pointed out the risk to your fuel system components, as well as the potential effect on detonation. Circuitsmith pointed out that, much to the chagrin of the EPA, combustion is not 100%. If it were, there’d be zero unburned hydrocarbons and if the metering was peerfact zero carbon monoxide.

I’d only like to add two things:

  1. lacquer thinner is not purified for combustion. It’s only meant to strip lacquer. It contains unknown misc junk with the acetone. You risk contaminating sensors and the catalyst with whatever is mixed in with the acetone, or at least whatever it turns into when compressed and combusted.
  2. everyone here is unanimous in recommending against trying this. Our reasons may vary, but we all agree. Don’t dump lacquer thinner in your gas tank. The results will be unpredictable and possibly expensive. Most of us here don;t even advocate aditives that are intended for automotive use except to squeeze a bit of life out of old beaters. None of us advocates adding things to the tank that were not made to go in there.

Most everyone on most boards has said the same exact thing. They never tried it. The very few who actually tried it said it actually worked and did not damage their car. While I am somewhat hesitant to actually try it, that conflict between the many who haven’t tried it and say it won’t work, and the few who tried it and say it worked is worth further investigation.

While I appreciate the advice as such, unanimous guesses are not all compelling.

Been busy on a major genealogy project; finally found the connection between the Aztec Emperors, and my wife’s native village. They are almost certainly descended from the Emperors, and that has taken my attention from this topic.

Had time to think it over. I may or may not actually try lacquer thinner. It is true that excluding myself all posters on this board unanimously reject it as a very bad idea. They also unanimously have never tried it, which is interesting, and seems to violate the usual standards of regulars, who at times really criticize those who speak out on topics where they have no experience. I remember once a man talking, I think it was about alignment, and a regular pointed out how foolish it was for a man who had never worked as an aligner to be disagreeing with those who had done it for years.

Another group that is unanimous, not on Cartalk, but on the many other forums on the Web which have discussed this topic, is the very small group which actually tried it. They were unanimous that it worked. Which is also interesting, and the only reason I am so much as considering trying it. Also, unanimously not one in that group reported car damage.

On a slightly related topic of running acetone, 2 ounces per 10 gallons of gas, one man reported damage, I think it was a much older Honda, but acetone regularly is different from lacquer thinner for several hours. Also, on the acetone forum, a warning about plastic tank liners on some models, such as the Prius.

And, one man took apart a fuel pump and soaked it in a double-strength acetone mixture for a very long time. He thought he could see a change in the parts, but his wife said she couldn’t. However acetone, even a weak mixture, all the time is different from one-time use of lacquer thinner.

However, with more thought, while I am not sure yet I will try it, I am sure I will only try it if I can get my hands on the same brand of lacquer thinner Kilmer used. It seems there is no standard formula for lacquer thinner, and to use a different brand would not be wise.

And, I will admit, it is scary.

My general attitude is, if I make a decision and it is wrong, to first, admit it’s wrong, and second, pay the costs. This is how we learn.

Part of my decision making process will depend upon if I can get my car back to the border. Because of the banditry near there, we are contemplating taking the bus back, it seems to be safer than individual cars, and rent a car while I am back. There is a limit to how long I am going to tolerate this CEL issue.

To my surprise, the light went out a few days ago. I erased the P0420 and drove quite a bit, around 90 miles just today, and no light. I will be driving another 100+ miles tomorrow.

I did order a new scanner today, from Amazon, to be sent to my mailing address in Texas, and my wife will bring it back in July. I want to see what is happening. An Equus 3140, with extra book and serial adaptor, it will be $216 total. I paid extra to get freeze frame, and live something or other, also the extra book for maximum information. Learning how to do this will be a good retirement project.

I cannot say for sure. I am going to GUESS that the catalytic grid somehow got crudded up during that long downhill run in second, with engine braking, and it took that long to clean it up.

Or, maybe a sensor got crudded up, who knows?

I am also GUESSing it will come back.

The problem with guessing is it doesn’t tell you much, except to try to understand what is happening without accurate data. If I get by until that scanner is in my hands, I will try again.

Despite the cynicism of many, I believe car mechanics are an honest lot. If they can save you some money by pouring magic potions in your gas tank or crank case they will do so or advise you to do it yourself. Few mechanics want you to eat a $1000 bill when they can offer you a $20 fix. A great example is a leaky radiator. If you have a minor leak and are not keeping the car for a long time most mechanics will skip the expensive radiator replacement and just add some “stop leak” into the cooling system. It is not a good long term solution but it works as a stop gap (no pun intended) measure, especially if you are planning on getting rid of the car soon.

History is replete with quacks offering solutions that people swear by. From Slick 50 to Pulstar sparkplugs to magnetic fuel line gizmos that “align” your fuel for better mileage. Remember The Vornado? It supposedly swirled the air/fuel mixture more for better combustion. There is no end to people who will take your money or pretend to be experts just to earn the false adulation of others.

When my trusted mechanic speaks to me I give him more credence than some dude on the internet and his groupies. Call me skeptical but when something goes wrong on your car you should generally fix it and not resort to voodoo and hearsay.

That’s my $.02

"Remember The Vornado?"

My father bought this one in ‘59, whole lot a swirlin’:

Irlandes, I too am guessing that the CEL will return.

While the CEL is either off or on, giving the illusion that the condition is either good or bad, the oxygen sensors themselves are giving out analog voltage outputs. The upstream, measuring the oxygen in the exhaust directly out of the cylinders, produces an output that varies as the cylinders fire, an output that resembles (but is not) a sinosoidal wave with lots of “noise” on it. The downstream one, measuring the oxygen level after the exhaust has been processed and “buffered” by the catalytic converter, has a higher average oxygen level and is relatively smooth. The ECU compares these two signals to determine if the converter is operating effectively.

Each of these signals can vary due to variations in the temperature of the catalyst as well as the actual cleanlines and constituants of the exhaust, which can vary based upon operatting and environmental conditions. Since the ECU is comparing two analog signals that are subject to other variations, and turning the light on or off based in this comparison, when the catalyst becomes marginal the light can turn on and off.

Catalyst become marginal primarily because its surface becomes contaminated with deposits from the exhaust. Its surfaces must come in contact with the nitrogen oxide molecules for it to do its job and seperate the oxygen from the NOx. Contamination prevents that from happening. But they do not clean themselves. What happens is that when the cat becomes marginal, the light can go on and off as outside conditions vary.

I agree with this last posting. Pretty much how I see it, that it is getting close and just bounced out and back in again.

I do not buy the groupie and voodoo statement in a previous posting. That is just another attempt by someone who has not tried it, to discredit all who said it actually worked for them, with no facts at all to base the comments.

I did think some more about this, yesterday when driving another 118 miles, light still out. I think there is a mechanic half an hour away who can probably get the converter off and back on again, and I could look at it and see what it looks like. Kilmer also said sometimes tossing it in soapy water overnight cleans it up.

To be honest, the more remarks which could be only described as sarcasm or insults, the more determined I am to give it a try, just to find out. Guessing as a basis for personal decision is somewhat normal. Most of us don’t have Ph.D.s in Automotive Engineering, and so we must make a calculated decision on important issues, based on limited information and our personalities. But, when people use sarcasm and insults on topics they absolutely do not know, that tends to get my goat. And, I tell you, voodoo and groupies crossed the line.