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Catalytic Converter: Avoid Unnecessary Repairs and Save Your Money


Glenn Stok applies his engineering background to solve technological problems and save money on car repairs. He shares his insight below.

Your check engine light is on. Your mechanic told you that you need a new catalytic converter, and it will cost you $900. Even worse, you need to have an annual inspection soon, and your vehicle won’t pass unless you get your catalytic converter repaired.

What the mechanic is not telling you is that it might just be a malfunctioning sensor, or the catalytic converter may be full of crud. Or you had filled up with a bad tank of gas that might have had water in it. It’s not that they are lying to you. They just really don’t know.

Most people think the only way to know for sure is to replace the catalytic converter, reset the check engine light, and see if it comes back on.

A few of my friends already had this experience, and I wish they came to me first. I found an alternate solution that worked twice for me already, and it worked for a few friends who listened to me.

What Is the Catalytic Converter?

The Catalytic Converter is in the line of the exhaust between the engine and the muffler. Its purpose is to reduce emissions from the exhaust with an efficiency level required by each state to control pollution.

The check engine light will come on if it’s below this threshold, determined by oxygen sensors that are located before and after the catalytic converter.

The sensors will trigger an Onboard Diagnostics (OBD) trouble code that’s stored in your car’s computer.


PO420 Diagnostic code

Catalyst System Efficiency Below Threshold

The Infamous Trouble Code P0420

OBD Code P0420 means the catalyst system efficiency is below the threshold. It‘s a generic code—meaning that it applies to all models of vehicles built since 1996.

That is the most common problem that makes the check engine light appear, although there are other problems that can occur. Each one has it’s own unique OBD trouble code. However, I’m going to tell you about an easy method of solving the problem that you should try first when your catalyst system is the culprit.

When a mechanic sees error code P0420, he or she will tell you that you need a new catalytic converter. This trouble code appears when the unit can’t sufficiently reduce the carbon emissions from the exhaust. However, a catalytic converter does not normally wear out. There is usually some other reason for their failure.

If you spend $900 or more to replace the catalytic converter, you may discover the check engine light coming on again later.

Some mechanics will tell you to try replacing the oxygen sensors first, to see if that solves the problem. There are two of them, one before and one after the catalytic converter. That's necessary to detect catalyst efficiency accurately.

A good mechanic might test by using a scope to diagnose the oxygen sensors or do a vacuum test or backpressure test, and just charge you a fee for the diagnostic service so that you can decide what you want to do as far as repairs are concerned.

So what do you do? Do you order numerous tests? Do you spend less money first for new sensors? Then if that wasn’t the problem, do you spend more money later on a new catalytic converter?

This Is What I Did When It Happened to Me

When my check engine light came on, I read the OBD trouble code that was stored in my car’s computer. I used an OBD diagnostic scanner that I bought on Amazon. You can get one for around $40. It works with all vehicles sold since 1996. Since then, all cars had to be OBD-II compliant.

The scanner has an LCD display that clearly shows the trouble code and indicates which device is malfunctioning in your car.

Some mechanics charge you just to do the same thing, and you’re still no better off. Catalytic converters should be diagnosed using proper methods involving more than a code reader.

However, I feel it’s a no-brainer to be able to read my car’s computer diagnostic trouble codes by myself, so I have an idea what’s going on.


I Have a Better Method

Clean out your catalytic converter.

How to Clean Out Your Catalytic Converter

There is also the possibility that the catalytic converter’s efficiency is compromised with oil deposits or other contamination, or water from some bad gasoline, or just plain gunk from the years of exhaust passing through it.

I found that there is an easy method to clean it out, after which it registers good catalyst system efficiency again. However, my method only works if the catalytic converter is malfunctioning due to contamination, as I just indicated.

So how do you clean it out? When you’re down to almost an empty tank, fill up a full tank with high-octane gasoline. Higher octane usually has more additives and especially better detergents that help clean out buildup of contaminants in the catalytic converter.

It might also be possible that you had previously gotten a bad tank of gas, possibly with water mixed in. That can reduce the efficiency of the catalyst too. High-octane gasoline might have better results with a cleaner mixture, or you can try getting your gas from a different gas station.

When you try my method, don’t fill up if you have a lot of gas in your tank. That will dilute the gas, and my method won’t work. You only need to do this once.

Drive until you’ve used up half the tank. When I did it, I just did my usual driving. In a couple of weeks, I used up half the tank. There’s no rush, no need to drive endlessly in one session. You’ve got time, don’t you?

After you’ve driven with high-octane for a couple of hundred miles, shut off the check engine light with the diagnostic scanner like the one I use.

Now, continue driving as usual for a week. Watch to see if the check engine light stays off. If it continues to stay off, you’ll know it was only a contaminated catalyst system. The high-octane gas actually cleaned it out.

It's worth trying to clean the crud out of the Catalytic Converter before spending a lot of money on repairs.

This method helped me avoid ever replacing the catalytic converter in my 20-year-old Honda. I’ve done this twice. Each time it lasted several years.

When it happened again, I filled up with high-octane and the problem was once again resolved. The light never came back on, and I passed inspection.

Is this legal?

You’re not doing anything to fool the system when you bring in your car for a state inspection. So why would this be illegal? You simply cleared the contamination and helped your car’s catalyst system perform to required specs again.

I'm sure it doesn’t help in all cases, but it worked for me and several of my friends who had the same issue.

What If It Doesn’t Work?

If the light eventually comes back on, then you have a more serious problem, and you might need that expensive replacement. However, there are still other less-costly causes for a trouble code P0420. Such as:

  • Failing oxygen sensor (remember that’s always a possibility).
  • Damaged wiring of the oxygen sensor connections.
  • Leaking exhaust system (exhaust manifold or muffler).

At least you’ll know for sure that you ruled out contamination, which I found to be quite common among my friends who saved money by following my method.

If you do end up getting a new catalytic converter, make sure you get one that meets your state's requirements, or else you may find the check engine light coming on again. The safest thing is to get an original manufacturer brand or one that is legal for all 50 states.

What Next, If You’re Going for a State Inspection?

After you reset the check engine light with the diagnostic scanner, the car’s computer will indicate a “Not ready” status until you’ve driven a while. That usually resets after 50 to 100 miles.

If you go for the inspection before the system indicates “Ready” you will fail the inspection. Before the inspection, attach your code scanner and check the status. Yes, it does that too. You’ll save yourself a trip.

Stay in the Know With the Same Diagnostic Scanner I Use

The way I look at it, getting your own diagnostic scanner is a small price to pay to do your own review of what's going on with your car.

Having a good mechanic you trust is still crucial, but it's nice to have the ability to know what's happening and to do something about it before spending a lot of money.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

Questions & Answers

Question: I do not have any engine light or "Service Engine Soon" light on my car, but the mechanic is telling me that CAT reading on the diagnostic tool is saying not ready. All test parameters for smog tests have passed except for the CAT reading. What would cause my CAT reading to register as unready?

Answer: If you recently reset your check engine light or if you disconnected your battery, then the status of various sensors will indicate "not ready." You have to wait until the ready status occurs. See the section "What Next, If You’re Going for a State Inspection?" in my article for full details.

Question: Will the check engine light (service engine soon) go off after the high octane remedy without the diagnostic tool?

Answer: The "service engine" light is not the same as the "check engine light."

The former is just a reminder that turns on every so many miles, based on the manufacturer's guidelines.

The latter is the one that warns you of malfunctioning emissions efficiency. This light does go off by itself, but only after a very long period when it detects no further errors from sensors. That does not provide a guarantee that it won't turn on again, especially if the situation is erratic.

Question: I always use 93 octane gasoline, even when I have no "check engine light" indicating a problem. What do I do to avoid unnecessary repairs and save money in terms of my catalytic converter?

Answer: You should only do that to see if it solves the problem before spending a lot of money on a replacement. Then you should go back to your normal octane rating.

If the light comes on again after resetting it, you may need your catalytic converter or the sensors replaced.

Question: I have a 2006 Subaru Outback. The P0420 sensor has gone on and off for about two years. My mechanic said not to worry. Now I hear the rattle. Is that indicative of catalytic converter issue?

Answer: The rattle you mentioned indicates that something might be loose. That might be a result of a damaged catalytic converter that is rattling. But it could be anything else. So you should have your mechanic put your car on the lift and eyeball the situation.

Question: My Catalytic Converter was plugged up and was so hot that it turned red. That caused the engine to shut down. I cut the converter off, and now the engine won't start. How can I troubleshoot issues with my Catalytic Converter?

Answer: You should never have removed the Catalytic Converter. It is there to control emissions pollution.

Tampering in such a way as you did can cause the engine to malfunction and maybe even fail to start. That is because the engine modifies the air/fuel mixture based on data received from the oxygen sensors that are before and after the Catalytic Converter.

In addition, you will now be registering an error code that will prevent you from passing your State’s emissions test.

If you followed my suggestion before you removed it, by using high octane gas for one tankful as I explain in my article, you might have been able to clean out the plugged up converter. But since you removed it, it’s too late for that now. You need to have a new Catalytic Converter installed.

Question: Is there a diagnostic tool for a 1991 model year Chevy S10 with V6 4.3L?

Answer: Since your car is a 1991 model, it may use the original OBD-1, which began being used on most vehicles in the US in the late 70s. You will need to check your vehicle’s owner's manual to see if it is one of those.

You can also look for the 12-Pin OBD-1 port that was usually under the hood. OBD-2, which began in 1996, uses a 16-Pin port located under the dash.

If your vehicle uses OBD-1, then you’ll have to purchase a diagnostic tool with a 12-Pin connector that is designed to read those older codes. You also need to buy an extension cord so you can sit in the driver's seat to carry out the required tests while reading the diagnostic tool.

A dual OBD-1/OBD-2 tool that includes everything you need, including the extension cord for the older models that have the port under the hood, can be found here: https://amzn.to/2M3muA5

Question: Another cause of a P0420 can be spark plugs, ignition coils, or spark plug wires going bad. I had an ignition coil and two spark plugs fail at the same time. Only the P0420 code came up, most likely from unburnt fuel in my front catalytic converter. These should also be checked and could be a less expensive repair than a catalytic converter replacement. Do you feel if those three things were replaced, that your high octane trick would clear the catalytic converter of the unburnt fuel remaining?

Answer: You are correct that other things can cause the P0420 code to appear, such as the problems you mentioned.

To answer your question, since those three things have been determined to be bad, they should be replaced even if no code is present.

Another cause of a P0420 error code could be water in the gas. Sometimes simply switching to gasoline from another gas station might help, as water might have gotten into the storage tanks of the first one.

The use of high octane generally works if no other issues are present. It's debatable why it works, as it did for me. It could be that merely using a different gas avoids any issues with bad gas, such as water in the fuel.

Question: I just had my catalytic converter changed as it was bad as it had all the symptoms (bad fuel consumption, loss of power, problem shifting gears, etc) since I got it replaced the car runs perfect like it did when I first got it. Though after driving it for 50 some odd miles the check engine light came back on and is showing the code for low efficiency. Is my car's new catalytic converter bad or could it be that the o2 sensors need to be changed?

Answer: You got that right. Since the light came back on with the same code, that means you probably never needed to replace the catalytic converter. The problem most likely is a bad oxygen sensor.

Question: My county in Texas does not require an emissions test, is it bad for the car to run with the catalytic converter running below threshold?

Answer: If it’s running below the required threshold with OBD code P0420, it means it’s putting out more pollution than desired. That does not affect performance, nor will it cause damage to the car.

Question: When you say high octane, can one use racing fuel?

Answer: Racing fuel is high octane. You’re right about that. But it’s not the high octane that is helping to clear the catalytic converter. It’s the fact that high octane gasolines usually have more detergents and less chance of water vapor mixed in. So the racing fuel will not help. Besides, some contain lead and are highly oxygenated. Two things I wouldn’t recommend for a regular vehicle.

Question: How does the P0420 OBD code affect the performance of the car?

Answer: The P0420 OBD2 code indicates that the efficiency of the catalyst system is below the threshold. But that doesn’t necessarily mean you have a bad catalytic converter.

The code will happen if the upstream and downstream oxygen sensors indicate similar readings. They won’t match if the catalyst system is within a proper threshold.

The car’s performance is only affected if the catalytic converter is extremely clogged, reducing fuel efficiency. You’ll feel it because it will be as if you have a clogged fuel filter, and the engine will tend to stall out under heavy loads.

Question: I have a 2011 Chevy Cruze. My car would not go over 50 mph on the highway. The CEL came on the next day and the diagnostic code was P2099. I took it to the Chevy dealer and was told it was the catalytic converter. It cost me $935 to fix it. Seven days later I got a call saying that they replaced the cat, but that didn't fix the problem. They said I would need to replace the rear one as well. Can both go bad at the same time with no symptoms at all? It ran fine prior to all that.

Answer: That P2099 trouble code indicates that the powertrain control module (PCM) is malfunctioning. In particular, that code refers to the PCM’s failure to control fuel trims to keep the air/fuel ratio correct. You wasted your money changing the catalytic converter.

Don't trust the mechanic giving you a code. At least look it up before spending any money. That’s why I recommend getting your own diagnostic tool as the one I mentioned in my article. It will save you from making these mistakes.

Question: How many catalytic converters does a Subaru have?

Answer: That depends on the model and year. You can find that information on the sticker under the hood.

Question: Can using a higher than usual octane do significant damage to your car?

Answer: The higher the octane, the higher the temperature required to ignite. So, no, it will not damage the engine because spark plug detonation is controlled by timing.

© 2017 Glenn Stok


Glenn Stok (author) from Long Island, NY on August 13, 2020:

NC - Try using a different gasoline. It might have water in it that can cause a low efficiency reading.

NC on August 13, 2020:

Thanks for your article. It's helpful to know that there can be other solutions.

I first got a P0420 code two years ago after a cross-country roadtrip. I drive a 2006 pontiac grand prix which had 150K miles at the time. I filled up with gas at the end of the trip and the check engine light came on. Mechanic #1 said it was the cat and gave me a quote for an OEM part: $2500 because my car has CA emissions standards.

Instead I went with mechanic #2 that put on an aftermarket cat for $500. They said it would solve the issue, even in a CARB standard car. But the light came back on right after replacement. Mechanic #2 then tested the efficiency and said the cat was efficient, but just probably not enough for my car's CARB standards. Then I got one O2 sensor replaced. Light still came back on.

I stopped worrying about the check engine light for over a year (same P0420 code, also an occasional P0302 code) because I moved to a state without emissions requirements. Now I am moving to another state that requires it.

Will I need to just get an OEM catalytic converter for the light to turn off?

Glenn Stok (author) from Long Island, NY on June 29, 2020:

Tracy Huelsman - Of course it can be something else, and even something simple, such as all the things I mentioned.

It's crucial you read through everything I suggested in this article (above) before you go back to the mechanic, so you don't waste money on unneeded repairs.

TracyHuelsman on June 29, 2020:

Hi, I have a 2007 Rav4. It threw a code for the catalytic converter and so I took it to a respected shop. The they cleaned it and replaced the muffler. That was last week. The light have come back on. Obviously, I am going to take it back. Is it likely that if the cleaning didn't work, that it is faulty? Or, could it still be something rather simple?

Glenn Stok (author) from Long Island, NY on June 19, 2020:

Aaron Null - It would be helpful to read this article. No need to repeat the answer to your question here in the comments.

Aaron Null on June 18, 2020:

We recently bought my mother in laws 2008 Cadillac cts, before we did so, my father in law had already replaced the passenger side cat and just before we got it i helped replace the driver's side cat and put on a new power sterring pump. The check engine light is on not flashing, i went to O'Reilly today to see what codes there were, i should have gotten a print out, but more or less it stated an issue with the converters. My father in law said something like we need to drive it like 100 to 150 miles and the light should go off? I have a scanner some where, should i just drive it this mileage distance or clear the codes and then see if it comes back on? The car runs fine and not any unusual smells noticeable.

Glenn Stok (author) from Long Island, NY on June 12, 2020:

Shailesh - I explain that in detail in this article. No need to repeat it here.

Shailesh on June 12, 2020:


I have a P0420 issue on my 2000 Honda CR-V. I used a fuel injection cleaner with the hope that it fixes the issue. I drove the car for a few miles and then used the OBS scanner to reset the check engine light. Since doing that I have driven the car at a good speed for 60+ miles and the light has not come back on. Do you think I can drive it for a few miles and risk a SMOG inspection? Would the OBD scanner tell me if the OBD monitor is ready or not?

Glenn Stok (author) from Long Island, NY on January 28, 2020:

Shane - California law requires all vehicles made in California after January 1st, 2009 must use a California regulated catalytic converter. That law applies even if you reside outside the state.

Shane on January 28, 2020:

Check engine light on, code for catalytic converter.

Mechanic said that since my car was made in CA, I would have to get a CA catalytic converter. 2001 Maxima, 124k miles. I have had several CCs changed over the years in various vehicles. This is the first I’ve heard of needing a specific state CC. We’re in GA.

Have you heard of this before?

He told me $2400.


Glenn Stok (author) from Long Island, NY on December 30, 2019:

Michele Solomon - Since you got gas from a different station, it's possible that it had water in the gas. That could affect the efficiency of the catalyst and cause the light to come on.

When your tank is almost empty, fill up from another station. Better yet, use high-octane on one tankful. That usually has more detergents that can help clear things up too.

As for the recalls, you need to know the OBD code to determine why the check engine light is on. Transmission and engine oil leaks are not related to the catalytic converter, but you should definitely get those things repaired. There is no cost for recall repairs.

Michele Solomon on December 30, 2019:

I have a 2014 Ford Fiesta, my check engine light came on after using gasoline from a different station. I received the code stating to change catalytic converter. I never had a problem before using different gasoline. Ford has a few recalls that I was wondering if it could be the cause of the light coming on. The engine runs fine. Ford recalls on transmission and engine oil leaks. Thanks for your expert advice.

Glenn Stok (author) from Long Island, NY on December 02, 2019:

Egbert Mntungwa - If you are asking about using standard gasoline—NO! You cannot use standard gas (no matter what octane) in a diesel engine. Regular gas ignites with a spark because it’s flammable. But diesel fuel, being combustible, requires heat from compression. The functionality is different (flammable vs. combustible).

Egbert Mntungwa on November 30, 2019:

Can you fill-up high octane in diesel engine?


TN on November 05, 2019:

Hi Glenn,

Many thanks for replying my question.


Glenn Stok (author) from Long Island, NY on November 02, 2019:

TN - Your Subaru has the catalytic converter integrated with the exhaust manifold. There’s one on the front of the engine and one on the back. That’s why the spare parts dealer asked you which manifold.

In case you’re wondering, in recent years many cars have the catalytic converter near the engine so they heat up faster. Catalytic converters are more effective after they heat up. That's why car manufacturers integrate them into the exhaust manifolds. You probably need to replace it as a combination single unit.

TN on November 02, 2019:

I have a Subaru Liberty 2013. The engine light of the car pops up intermittently and looses power (AWD). I took to a local mechanic for a computer scan and it came up with low efficiency catalytic converter. If I reset the battery, it goes away and comes back after some time.

I called a spare parts dealer for a used catalytic convertor and he asked WHAT MANIFOLD THE CONVERT WAS. I didn't know what he was talking about and asked the local mechanic. The mechanic has no clue either what it means.

Can you please advise how can I ensure I get the right one and tell the exact specifications to get the spare part.

Glenn Stok (author) from Long Island, NY on October 14, 2019:

S-parent 86 - It’s true that you needed to get the sensor replaced due to the P0341 code. But your more serious problem, causing the car to die, still needs to be found by a good mechanic. It’s most likely not related to an emissions problem.

S-parent 86 on October 13, 2019:

I know this is bad but i have a 2003 Honda Accord ex 2.4L. Ive been driving it for about 2 years with the check engine light on. A few months back it started dying when it got warmed up at stop lights or anytime i would slow down or turn corners. I would have 2 push in the clutch and keep revving it up. 1 place said it was my timing belt and they said the belt alone was $106 through them. Of course i didn't buy it cause i know my car doesn't have a belt. Took it 2 a different auto car place they read me off the code. P0341. I also have 2 OBD readers and both confirmed it was P0341. I replaced the Camshaft Position Sensor, Bank 1. It got worse, then just died while driving and its been sitting ever since. Why would 3 OBD readers say the same thing and replacing the part make it worse and die?

Glenn Stok (author) from Long Island, NY on September 17, 2019:

Amanda M - It’s definitely worth a try before you spend more money.

Amanda M on September 17, 2019:

Hello! I drive a 2006 Dodge Avenger. My engine light was on and off for a couple weeks then stayed on long enough that I took the car in to get checked. $111 for a diagnostic and they told me it was the code for the oxygen sensor/catalytic converter. On their advise, I had the oxygen sensor replaced first. It cost me hundreds! The light came back on a week later and has stayed on for three days now. Does this sound like a good candidate for your high octane gas trick?

Glenn Stok (author) from Long Island, NY on September 01, 2019:

Joseph K - Other things can cause the P0420 error. You could have an exhaust leak. Check for damaged exhaust pipes. Or you might have damaged wiring or connectors to the O2 sensors.

Joseph k on September 01, 2019:

I have this 2001 toyota camry with 2.2 le troubled code P0420.I installed 3 catalytic converters but P0420 will not disappear.All O2 sensors are new,new egr valve and new vacuum modulator.VSV valve is also new.Whats going on here,can somebody help,will appreciate.

Glenn Stok (author) from Long Island, NY on August 31, 2019:

Maria - Same answer I just gave to Aimee Guidry below.

Maria on August 31, 2019:


Toyota Camry hybrid 213000 miles engine light showed on, got checked was code P0420 after 2 days turned off it self with out doing anything, do I still have to get fix or wait until light shows again, please what is you recommendation?

Glenn Stok (author) from Long Island, NY on August 30, 2019:

Aimee Guidry - The fact that your light keeps going off, then on again, indicates the possibility that you only have an intermittent problem. That could be faulty O2 sensors, or it could be the wiring. Those are much cheaper to repair than getting a new catalytic converter.

Aimee Guidry on August 29, 2019:

Hi. My check engine light came on. We tested it with a scanner. My mechanic told me I needed to change the cat converter (bank 1). I read that the o2 sensors have the same code. Do I guess which one it is? Also, the check engine light comes on, goes off, them comes back on. Thanks!

Glenn Stok (author) from Long Island, NY on August 27, 2019:

Indra Fauzi - If your light keeps coming back on, that is an indicator that you do have a problem that needs to be fixed.

Indra Fauzi on August 27, 2019:

Hey Glenn, I have a 2006 Cryssler Pasifica and i'm getting the P0420 code. I already used your method to fill the gas with high octane for 3 times. My car no loss of power, no strange odors, or any rattle noises. When i disconnecting the battery the engine light comes off. But, unfortunately The engine light comes on again. What do you recommend with this case?

Glenn Stok (author) from Long Island, NY on August 16, 2019:

Carla - You’re asking the wrong question. I think you are being taken advantage of. You said you were told it “could be” the catalytic converter. I would not trust those words. It either is, or it isn't.

If you want to know for sure, you should get your own diagnostic scanner, such as the one I mentioned, and check the diagnostic code your car is reporting. Paying under $40 for your own scanner is a lot cheaper than $1000 for a replacement you may not need.

Besides, I never heard of a bad catalytic converter keeping the car from going over 10 mph. As for the noise, it might just be a hole in the muffler.

Once you rule out the catalytic converter with your own scanner, go find an honest repair shop to figure out why you can’t go over 10 mph.

Let me know the results. Good luck with it.

Carla on August 16, 2019:

My car started making a loud noise and I couldn't go over 10 miles. Was told it could be the catalytic converter. Is this hard to fix? Can it be done in 1 day? Thanks!

Dipen on August 15, 2019:

Thanks, Glen. I am going to have the dealership run the diagnostic test this time. I hope nothing major comes out of it. Thank you again for your response.

Glenn Stok (author) from Long Island, NY on August 14, 2019:

Dipen - It sounds like you never needed a new catalytic converter. That usually is the case when people continue to have problems afterwards.

Since you had all the related parts replaced and still have the problem, it might be faulty wiring.

You need to have your mechanic perform a through test to locate the true problem. Maybe find another mechanic you can trust, one who won't sell you unnecessary work. Good luck with that.

Dipen on August 14, 2019:

Glenn, the code that I am getting is P0138 - O2 Sensor High Voltage Bank 1 Sensor 2. I am not sure what can be done to fix this. My car is Honda Accord 2006. The initial code I got was P0420 which led me to replace both the catalytic converter and the oxygen sensors. One of the sensors turn out to be faulty and gave the code P0139 so my mechanic swapped it with another new one. Now after about 4 days the light came back on with the code P0138. Please help!

Glenn Stok (author) from Long Island, NY on August 14, 2019:

Dipen - Check the diagnostic code with a scanner as I suggested. That will tell you why the light is on.

Dipen on August 14, 2019:

I got both the oxygen sensors as well as the catalytic converter replaced but the check engine light came back on after 4 days of driving. What do you recommend in this case?

Glenn Stok (author) from Long Island, NY on August 07, 2019:

Tyler Dunlap - The fact that your light keeps resetting and then coming back on indicates you have an intermittent problem. It might be a failing O2 sensor. However, the actual cause of the problem can only be determined by a mechanic who can check your vehicle.

Tyler Dunlap on August 07, 2019:

Hey Glenn, I have a 2007 Suzuki Grand Vitara and i'm of course getting the imfamous P0420 code. Now i do a lot of highway driving, roughly 1500 miles a month unfortunately. Now i have no loss of power, no strange odors, or any rattle noises. The engine light comes on and off roughly every 100-200 miles i drive. It doesn't return until roughly 3-4 days after it turned off. Now i'm assuming it's probably a bad O2 sensor and just wanted your opinion on my situation. I'm going to try your method with the higher octane gas later once my tank is low on fuel to properly test this method.

Thank you! on August 06, 2019:

I’m in the middle of trying your method with using the high octane gas and turning off the check engine light after half the tank has been used. I’ve gone about 50 miles so far and no light yet. Fingers crossed that I’m in the clear because inspection is next month and a new cat converter will cost me $1100.

Glenn Stok (author) from Long Island, NY on August 04, 2019:

tcysimon10 - Since your light went off by itself, you might just have an intermittent problem. Possibly with one of the O2 sensors. I would hold off on the expensive replacement of the catalytic converter since the light went off on its own. If it comes back on, you could try the O2 replacements first.

tcysimon10 on August 04, 2019:

Hi Glenn, my 2006 Sienna engine light came on last month, OBD scanner displayed P0430 (catalytic converter bank 2). I did try using a full tank of high 91 octane gas but the engine light came on last week again with the same issue. TOYOTA mechanic did a diagnostic test, recommended to replace the converter. However, the engine light came off this morning, should I go to replace it because I have make an appointment to replace it next week. If the O2 sensor fails, will the OBD scanner indicates it is the converter problem still ? Should I replace the O2 sensor first, thanks.

Glenn Stok (author) from Long Island, NY on June 24, 2019:

Tom McCullough - You do have to drive a good distance before the computer resets the “ready condition” after shutting off the light. And yes, disconnecting the battery will cause the light to reset. But after the computer detects the problem is still there, the light will come back on.

The Sea Foam additive is an engine cleaner. But if that didn’t work, then the method I described is worth a try before spending money on a repair.

Remember that you will need to reset the light and then drive a distance for the computer to detect the status. But driving 110-115 mph, as you said, is ridiculous and dangerous. Whatever you read that suggested that should not have taken seriously.

Tom McCullough on June 22, 2019:

I have a 2011 QX56 with 61K miles and the dreaded service engine soon came on. Autozone said it was the catalytic converter code left side. I read on another post to add two cans of Seafoam to the tank when it's half full. Then "drive it like I stole it" for a while. I drove 85 miles last night and was driving excessively fast....like up to 110-115 mph. I'm still alive and my engine light is still on. I will try the other tricks like running the tank to E then filling up with high octane. Will the light automatically go out or is that something I would have to reset? Would disconnecting the battery and reconnecting make the light go out?

Glenn Stok (author) from Long Island, NY on June 16, 2019:

Sara S Mccroskey - Since you had recent work done, replacing the Catalytic Converter, you may have an opening in the intermediate pipe leading to the muffler if it wasn't done well. It may have been rusted and overlooked by the mechanic when replacing the Cat Converter. That would cause the same noise as a hole in the muffler.

If the light is still coming on, it’s possible that your sensors may have been bad instead of the Cat Converter.

Sara S Mccroskey on June 15, 2019:

Unfortunately, i didn't see this until I replaced my CAT. I have a Jeep Wrangler 2008. Now today driving into work, it sounds like my muffler is bad. It doesn't happen in 1st gear at all, only 2-6. My check engine light goes on then off right away. Any suggestions as to what is making that loud noise, or why my check engine light keeps going off. Thank you so much for any help.

Glenn Stok (author) from Long Island, NY on June 05, 2019:

Cinndy55 - At this point you’ll never know if you really needed the replacement. It’s too late to try my method.

Cinndy55 on June 04, 2019:

Well, I got the P0430 code and I was charged more than the $800. Another shop told me it was my Bank 2, 02 sensor downstream, passenger side on my 02 Nissan Pathfinder converter but the shop doing the repairs said Bank 2 is the engine side. So, they did the repairs but I'm still having a slight jerk in 1st gear. My brother said its the 02 sensor on the passenger side Bank 2 just as the other shop stated. Did I, a 63 year old female got ripped off! Thanks

Glenn Stok (author) from Long Island, NY on May 23, 2019:

Jennifer - If the light keeps coming back on with code P0420 after the O2 sensors were replaced, then it’s probably the catalytic converter. But make sure your mechanic confirms it’s not bad connections for the sensors, or burned wires.

Jennifer on May 23, 2019:

Well hello again. So I did the high octane gas thing...used full tank in 2 days... light still on, but more importantly, still having that jerky hesitation feeling when accelerating. Code is P0420.... has been the same code the last 3 or 4 times its come on. Replaced both o2 sensors and 1 catalytic converter. Mechanic reset me after last viist with no findings of any problems... but if course a few days later, light is back on, same code. One interesting thing, I didnt have the jerky hesitation thing until the light came back on a few days ago (about 1 week after last mechanic visit). 2015 Chevy cruze 2lt. 123k miles

Glenn Stok (author) from Long Island, NY on May 22, 2019:

Jennifer - It’s impossible to say without knowing the kind of car you have or what the codes are that your getting.

Jennifer on May 22, 2019:

Thanks Glenn, i will try! I did replace both o2 sensors but I think my mechanic mentioned possibly having 2 catalytic converters...? I only replaced one so far. But if the other was bad, when they tested o2 sensors would those numbers have read accurate?

Glenn Stok (author) from Long Island, NY on May 21, 2019:

Jennifer - There are two O2 sensors. One before and one after. It could be the other one that's bad, or it could be the catalytic converter itself. Try my method first.

Jennifer on May 21, 2019:

2015 Chevy cruze 2LT... I've had the o2 sensors replaced, then the catalytic converter (is there more than 1? I think my mechanic mentioned we're only replcing 1 to start?) Anyway the light keeps coming back on, mechanic ran tests and said o2 sensors and cat are reading accurately, reset the code, and a few days later its back on (I drive a good 700 miles per week). I do notice a rough idle at times and a slight hesitation when stepping on the gas. Ive also used a $30 recommended cat cleaner from the auto parts store. Any suggestions? Should i just try the high octane gas thing? Would that help with the rough idle and hesitation though?

Glenn Stok (author) from Long Island, NY on May 01, 2019:

Laura Homsey - Thank you for posting that feedback on the results of using my method.

Laura Homsey on May 01, 2019:

Glenn! Because of your article we saved $1,500! We were told by a garage that our cat converter needed replacing, but something told me there was something else to try. Luckily I found your page, and diligently followed your advice. Today we ended up passing our State Inspection with flying colors. I'm also happy to have a diagnostic code reader now to do my own research when problems arise. Can't express how grateful we are to you, thanks again!

Glenn Stok (author) from Long Island, NY on April 18, 2019:

Stephanie - That all depends on what is causing the reading. The issue could simply be that the sensors are faulty, and that will give the same code as if the catalytic converter was bad. That's why you need to have an honest mechanic review it.

But if you are told you need a new unit, try my method first. It saved me the cost of having any work done and it worked for several of my friends already too.

Stephanie on April 18, 2019:

If my car already failed inspection bc of the engine light on bc of catalyc converter will this device still help or do i now have to get my car actually fixed?

Glenn Stok (author) from Long Island, NY on April 01, 2019:

Rob, Octane does have a higher combustion level. That is, it ignites at a higher temperature. You are absolutely correct.

Rob on April 01, 2019:


Above, in the Q and A section, you told someone with a question that running high octane fuel will damage their engine because it can cause premature detonation. Octane is a rating of resistance to detonate and the higher the octane, the higher the resistance to detonation, the exact opposite of what you said. This is why higher octane fuel is recommended (if not required) in vehicles with turbos, superchargers or high compression heads or pistons. This leads me to believe the rest of what you have written is BS, as well.

Glenn Stok (author) from Long Island, NY on March 23, 2019:

Leroy polczynski - It looks like you were screwed. Do you know for a fact that they actually installed a new catalytic converter? In addition, a reputable mechanic would know if the new sensors they installed were compatible. You need to go to a reputable mechanic to have it checked out. There is no way to do that remotely, which would only be guess work.

Leroy polczynski on March 23, 2019:

Had a problem with o2sensors. Found out the aftermarket and factory don't like each other. Dealer changed them out now have the infamous cat code,p0240! Could this just be because o2s weren't talking to each other. Replaced cat last year with a whole new exhaust system

Glenn Stok (author) from Long Island, NY on March 22, 2019:

Diosa1202 - Did you try my method first? That may help clean it out and avoid having to replace it.

Diosa1202 on March 22, 2019:

I had an O2 sensor reading and my mechanic changed the sensor 2x but it keeps getting the check engine light come back. Now he is telling me i need to change the converter. I feel that he hasn’t run enough tests to know this is what it needs for sure. I drive a 2006 Jeep Liberty. What are your thoughts?

Stephanie Gordon on March 20, 2019:

Thanks for your speedy reply. My upstream O2 sensor is new. I haven't gotten an error message for the downstream one yet. I might try the engine cleaner since it costs less than a tank of high octane gas. I am trying everything I can before replacing the catalytic converter, since it is so expensive.

Glenn Stok (author) from Long Island, NY on March 20, 2019:

Stephanie Gordon - Since it stayed off until you filled up again, assuming you went back to the usual lower octane gas, maybe you just need to run it longer with high octane. You could always try cleaner, but the high octane does the same thing.

If the light continues to come back on with the error code, then you might have faulty sensors. Or worse, it actually could be that the catalytic converter needs to be replaced.

Stephanie Gordon on March 20, 2019:

I followed your suggestion and ran a full tank of high octane gas in my car. The light stayed off until I filled up with gas again, and then the PO420 error message came up. Would you recommend an engine or catalytic converter cleaner as a next step? Maybe #1 and 2 fuel injectors are dirty and causing the reading. Further suggestions will be appreciated very much. Thank you.

Glenn Stok (author) from Long Island, NY on March 08, 2019:

Stephanie Gordon - You may as well wait to see how it is after your trip with the high octane gas. If the light stays off after that, then you’ll know it was not either of the O2 sensors. Please post your results here. Have a good and safe trip.

Stephanie Gordon on March 08, 2019:

I have a 2002 Toyota Camry with 317,000 miles on it. I have recently gotten a P0420 error message. The dealer in my area told me to replace my catalytic converter. I have already replaced the spark plugs, # 3 and #4 fuel injectors, and the O2 sensor-upstream. I have done all of this work myself, as I am retired and trying to save on car repair bills. I am going to try running a tank of high octane gas through my car next week on my drive from VA to GA. I hope this will do the trick. Do you recommend replacing the downstream O2 sensor as well? My car is running fine after it warms up for a few minutes. Thank you so much for your informative articles and advice.

Glenn Stok (author) from Long Island, NY on February 20, 2019:

KayRhea - It has nothing to do with driving over 50 miles. That is only necessary to get all the signals to reach “ready” state after the light has already been turned off, as I explained in this article. Since your light is still on, the mechanic didn’t fix the problem. You need to go back and get them to do the job right.

Did you get your own code reader, such as the one I mentioned? That is the only way you’ll know what the computer is complaining about, and then you’ll know what to expect the mechanic to be correcting.

KayRhea on February 20, 2019:

I just had my catalytic converter put on my 2013 Ford Focus SE and the check engine light is on. Would it have to do with me not driving my 50-100 miles?

I have had to replace fuel pump.. O2 sensors and vapor canister. I have no idea what more it could mean and I spending way too much for the check engine light to keep coming on. PLEASE HELP

Jhope on February 17, 2019:

Its works people! My engine light came on, autoshop said it would cost 3k to replace the “cats”. I didnt get the device Glenn described, but my light went off on its own with the high octane gas like Glenn described. I could only find 91 premium gas, so still worked. I have a 2002 4runner. Thank you Glenn!

cingram on January 17, 2019:

Thank you for offering this helpful information. My Camry has a P0420 code set though the MIL (or check engine light) is NOT on. I'm going to try your method and see what happens. Thanks again. You provided a useful possible solution as well as quite a bit of other useful information.

Glenn Stok (author) from Long Island, NY on January 16, 2019:

Natalie RG - The dealer who sold you that car lied to you. Never trust buying a car when the check engine light is on. It will never pass inspection. It's too late since you already make the purchase.

Your only option is to sue the dealer if you want to go that route. But if you don't have the paperwork to prove the condition upon purchase, you're out of luck.

Natalie RG on January 16, 2019:

My 2008 Ford Expedition is giving OBD 304, 308, and 420. The dealer says that I failed to maintain the catalytic converter plugs which led to raw fuel dumping into it. Well, the check engine light has been on since I bought it and they explicitly told us that it was just an error with the o2 sensor and we shouldn't worry about it as everything was working fine. We've stayed up on all of our maintenance, but the warranty won't cover it because they are saying it's our fault and we should have brought it in sooner. I brought it in as soon as the vehicle's performance changed.

What do you recommend?

Glenn Stok (author) from Long Island, NY on December 26, 2018:

Quentin R. - Thanks for confirming that my method worked for you. You sure kept excellent records as proof!

Quentin R. on December 24, 2018:

Great article! I followed everything the article suggested after failing emissions with trouble codes P0420 & P0430 on 12/4/18 at 15:19:16 but 20 days later I went back for a retest on 12/24/18 at 14:29:26 and got the paper in my hand signed with the overall results: PASS. I also, before and after pictures of the results, just in case you were as skeptical as I was.

Glenn Stok (author) from Long Island, NY on December 18, 2018:

Carlosbell - That has nothing to do with the Catalytic Converter. Your check engine light would be on if you have any emissions issues, in which case you can read the OBD code to see what it is.

Carlosbell on December 18, 2018:

Hi Glenn,

Wish I would have seen this article before now.

So my 2006 Nissan Altima is having issues accelerating. While parked, if I push the gas pedal to the floor, it will barely its 2x rpms.

Was wondering if you believe that to be a catalytic converter that needs to be cleaned or replaced.

Thanks in advance

Glenn Stok (author) from Long Island, NY on December 04, 2018:

Mozee - You should report that machanic for fooling you. It’s illigal in any state to charge for a new one and then just clean the old one. You need to check with Toyota about your question if there is more than one. In any case, try my method before spending any more money.

mozee on December 04, 2018:

I had a catalytic converter replaced on my 09 Toyota Sienna. I explicitly told the mechanic I wanted a new converter installed which he agreed to. After 11 months , the engine light reappeared and the mechanic says the other converter has gone bad and needed to be replaced. My instincts told me to get a 2nd opinion and I went to another mechanic recommended by Emissions. He took pictures of my converter and it showed instead of replacing the converter, with a new one, they merely cleaned it and welded the pipes. I was told this is illegal in the state of MD. Presently I can't afford to have it replaced since I have just paid out a lot of money. My question is are there multiple catalytic converters on my van, and could the welding that was done cause the other to go bad?

Glenn Stok (author) from Long Island, NY on December 03, 2018:

Danielle - The deceased gas mileage has nothing to do with the catalytic converter. You might have other problems developing.

Remember to try my method if the light does come back on before you spend money on a new one.

Danielle on December 03, 2018:

I have a 2005 Nissan Altima. My check engine light came on, took it to my mechanic and he says my catalytic coverter needs to be replaced. He just doesn’t know how long I have. So he cleared the check engine light and said to wait and see if comes back on. If it comes back on in a day or two that means it needs to be replaced right away. If it doesn’t come back on that means I have a while. It hasn’t come back on and that was a month ago. Meanwhile my gas mileage is decreasing. And I’ve noticed I have less power while accelerating than I did a few months ago. Does that mean he’s right?

Jamie on December 25, 2017:

Still working my way through the tank of premium gas. I filled it all the way up (tank capacity is 12.3 gallons, I put in 12.16 gallons). Hoping that it will clean whatever was causing the light to go off out. As I mentioned, my light was going on and off for about a hundred miles and was off prior to me filling the tank with high 91 octane fuel. The light has not come on again.

Jamie on December 16, 2017:

Glenn, thanks for clarifying. It's for a 2008 Civic Hybrid that has always been carefully maintained. I'll stick with the 91 and report back.

Glenn Stok (author) from Long Island, NY on December 16, 2017:

Jamie - Too high might blow the engine if it's not designed for racing. Use just all 91. It worked for me. Just don't dilute it. Fill on a close-to-empty tank.

Jamie on December 16, 2017:

Glenn, thank you for your response! I am going to be trying your method. I've got a P0420 code that has been going on and off (now it's off) for the past two days. There is a Union 76 near me that has 101 octane racing fuel. I was thinking of getting about 9 gallons of 91 and then 1 gallon of the racing fuel. Would that be okay or should I just stick with ALL 91?

Glenn Stok (author) from Long Island, NY on December 16, 2017:

Jamie - I was referring to the highest octane you can get in your area. That varies. Typically in the U.S. octane ratings can be as high as 91–94 for premium gasoline.

Jamie on December 15, 2017:

How high is high octane? Are you saying something like 101? Or 91?

Nell Rose from England on October 13, 2017:

Yes my brother said the same! All the cars are sealed in now! long time gone is when you could tie a pair of tights round the watsit! LOL!

Glenn Stok (author) from Long Island, NY on October 12, 2017:

Nell, You're probably better off that you don’t drive. These days cars require a whole different type of maintenance. When I was young, I used to take care of oil changes and other things myself. There was always room to work. Now I can’t even squeeze my hands in far enough to change a dead headlight bulb.

Nell Rose from England on October 12, 2017:

Very useful and helpful, and something I know nothing about and as I don't drive I can't really comment. But this will be really useful for someone who has that dreaded 'need a new one' syndrome.

Dora Weithers from The Caribbean on October 06, 2017:

Very helpful, useful information, although I would engage someone else to help me follow the instructions. Thank you.

FlourishAnyway from USA on October 04, 2017:

Extremely useful information, Glenn! Thank you!

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