Another look at P0420 cat efficiency

Back in 2011, my P0420 code, cat efficiency issues, came on after a 17 mile downhill run with engine braking. I found a Youtube video by radio personality Scott Kilmer which said to put lacquer thinner in the gas tank.

I did a lot of Googling. Everyone who tried it said it worked, without damage to the motor, except the owner of a much older Honda who said it did induce damage to the fuel system.

Everyone who had never tried it said it would damage the fuel system.

In my case, after some weeks the code went away. Not long ago it came back. I reset it, and drove to the border and back again, some thousands of miles, with no further code. The output sensor runs around 0.7 which does seem a bit high.

I found a Youtube video by a person who said he tried lacquer thinner and it does go out. But, after not long it comes back on.

Just now on Sienna Chat, a man said he has the same results with “Chevron Fuel Injector Cleaner with Techron (the latter is reportedly important based on other posters)”. Goes out long enough to pass verification, then comes back on. He says he has done it three years just to get past verification.

That would probably be less scary since at least Fuel Injector Cleaner is intended to be used in car fuel systems. And, the use of an automotive designed cleaner instead of lacquer thinner is the only reason I am bringing up this dead horse.

In my case, I will eventually install new output sensor. And, if I had the problem the man had, where it comes back on, I would probably replace the cat after while, if sensors did not fix it. But, that is my viewpoint.

Others don’t care since the light comes on at 95% efficiency.

Irlandes: I’ve found the old Toyota Sienna’s like yours can get finicky with the PO402 code - either after the cat and all sensors have been replaced (with aftermarket parts instead of Toyota OEM), or simply with age.

If it’s your own vehicle and all you’re interested in is to suppress the P0420 code, the use of a spark plug non-fouler should do it. There is lots of “how to” info available on the net. Some even put a bit of steel wool first before screwing in the O2 sensor (to help further deaden the airflow).

Lacquer thinner is basically acetone and the only effect it may produce is destroying your fuel pump and any rubber fuel lines. It will not restore a dead catalytic converter or oxygen sensor.

“But it made my CEL go off” No it didn’t. As the borderline converter efficiency swings up and down, the light comes and goes and over time will stay on more than it goes off…


If you want to clean your injectors, I recommend this product. I’ve used it with great success.

You have to have a canister fuel cleaner tool, though. This is potent stuff and the engine actually runs off of it. This is NOT the stuff you pour into the tank.

I don’t recommend using fluids from the auto store in your paint, and I don;t recommend using fluids from the paint store in your auto.

Even if tghe base chemical from a paint store and an auto store are the same, their different applications can mean they have different levels of purity and different additives. Just as regular glass and fused quartz are both SiO2, regular glass will self destruct if used in a quartz-halogen bulb.

While I’m not an advocate of additives as permanent fixes, if you’re going to use one at least use one made for the purpose.

And I also recommend taking advice from a mechanic rather than a “radio personality”. The former knows cars, the latter knows entertainment. Use the proper expert for the proper questions.

Is the thinner, in order for it to fix this DTC problem, does it supposedly clean the sensor, or does it clean the cat? I don’t see it helping the cat much, and it could well damage the cat if too much thinner was used; but it might help clean the sensor I suppose. But if this is the intention, why not just remove the sensor and try cleaning it with some thinner yourself, on the bench? Might well work. Then you wouldn’t risk damaging the cat or the rest of the fuel system.

George, perhaps the radio personality can tell us.

Allow me to suggest that if I had a car that stored a cat low efficiency code whenever I descended a long hill using engine braking, I’d wonder about the valve stem seals. The engine could be drawing in oil under high vacuum conditions, and when sustained could be depositing carbon on the sensor and/or platinum-palladium.

You can call e-mail Scott at . He does answer e-mails.

And, he already told you in the youtube video he made on this topic.

Most of the arguments here are simply re-hashes from 2011. As I said several times I Googled for hours and everyone who tried it said it worked and didn’t hurt the car (except the old Honda). And, everyone who never tried it said it couldn’t work and would hurt your car. Pure science?

The only reason I posted this is because someone said it will work, sort of, with a substance designed to be run in a car, and not something from the paint store. I thought that would be useful, and it certainly was not my intention to re-hash the same tired arguments about lacquer thinner.

Yes, for the person who asked it does indeed clean the cat, just as my cat cleaned itself by being driven. No, it is not to clean the sensor.

Scotty also says you can clean it if you can get it off the car, by tossing it in soapy water overnight.

As far as my Sienna, some time ago it was posted on this board that the computer had a software problem which can cause this. Apparently, the specs are set too tight. I think the computer costs almost as much as a new cat, though.

I had the same thought as someone mentioned about oil leakage during that down hill run. According to my Equus live data scanner, the gas to the injectors is apparently turned off during downhill braking. Which means almost any quantity of contamination will clog things up because the cat is going to completely cool down, I think.


Here’s that bulletin you were just talking about

If it were my car, I’d check what part number your PCM has.

Just something to think about, before doing anything exotic . . .

I will say, from personal, professional experience, that TSBs in general very often fix the problem, and software flashes (or PCM replacements) to fix a SPECIFIC problem do work.

Of course, there’s always the possibility that P0420 is being generated because the oxygen sensors have degraded, or the cat itself has degraded . . . possibly due to oil consumption, misfires, blown head gaskets, etc. Mind you, these events may have occurred in the past, but shortened the cat’s normal lifespan . . .

That bulletin is a few years old. Perhaps it is now possible to flash the PCM, versus replacement.

It’s a few years old?

Website folks, it would sure be great if you could add the year to the date posted. It’d prevent a lot of zombie threads that keep coming back to life.

I’ve mentioned before my theory that the cat simply cooled off during the descent, then later the computer did a cat test on a still cool converter.
They have to be hot to work efficiently.


Let me clarify

This thread is NOT a few years old

However, that technical service BULLETIN I posted is a few years old. It’s probably still valid, however.

I posted it because I’m trying to help @Irlandes

This is NOT a zombie thread that’s come back to life

Thanks for the clarification db. I definitely misunderstood. We get too many zombie threads.