Catalytic Converter: Avoid Unnecessary Repairs and Save Your Money

Updated on October 4, 2017
Glenn Stok profile image

Thanks to his technical background, Glenn Stok is skilled at solving technological problems, which has helped him save money on car repairs.

Your check engine light is on. Your mechanic told you that you need a new catalytic converter and it will cost you $800.

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. 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.

This Is What I Did When It Happened to Me

When my check engine light came on, I read the Onboard Diagnostics (OBD) trouble code that’s stored in my car’s computer. I used an OBD diagnostic scanner that I bought on Amazon. You can get one for under $20. It works with all cars 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.

I feel like it’s a no-brainer to be able to read my car’s computer diagnostic trouble codes by myself, so I know for sure what’s going on.

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 before and after the converter.

PO420 Diagnostic code

Catalyst System Efficiency Below Threshold (Bank 1)

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.

This 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 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 $800 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. This is necessary in order to properly detect catalyst efficiency.

A good mechanic might also do proper testing 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?

I Have a Better Method

There is also the possibility that the catalytic converter’s efficiency is simply compromised with oil deposits or other contamination, 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? Easy. High-octane gas can blow out the contaminants. When you’re down to almost an empty tank, fill up a full tank with high-octane gasoline—the highest grade available. Don’t fill up if you have a lot of gas in your tank. This will dilute the high-octane gas and my trick 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.

As I mentioned, I’ve done this twice already. The first time lasted several years. When it happened again, I filled up with high-octane and the problem was once again resolved. I passed inspection and the light never came back on, even long after that.

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 blasted out the contamination and helped your car’s catalyst system perform to required specs again.

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 still 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 States 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. This 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

Professional U480 CAN OBD2 OBD II Car Diagnostic Scanner Engine Code Reader Tool
Professional U480 CAN OBD2 OBD II Car Diagnostic Scanner Engine Code Reader Tool

Now that you have your own diagnostic scanner such as this one that I use, you can always check your emission system’s status yourself. No need to pay a mechanic for a needless reading. Never get sucked into a repair job you may not need.

The way I look at it, this is a small price to pay to be able to do your own review of what's going on with your car. Having a good mechanic you trust is still important, but it's nice to have the ability to know what happening and to do something about it before spending a lot of money.


Questions & Answers

  • My Chevy S10 V6 4.3L is a 1991 model. Is there a diagnostic tool for this year?

    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 owners 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 drivers 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:

  • 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 test are already passed except for the CAT reading. What is the problem?

    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.

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

    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. This does not provide a guarantee that it won't turn on again, especially if the situation is erratic.

    If the high octane remedy that I discuss in the article works with clearing the catalytic converter, then the sensors will stop sending an error code to the computer. However, as I said, this takes a lot of time.

    You'll be waiting for the light to go off, and this may take weeks or months. It's best to turn it off with the diagnostic tool and monitor the status for a "ready" condition before you bring the car in for an inspection.

  • My Catalytic Converter was plugged up and was so hot that it turned red. This caused the engine to shut down. I cut the converter off and now the engine won't start. Any suggestions?

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

    Tamperiing in such as way as you did can cause the engine to malfunction and maybe even fail to start. This 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.

  • I always use 93 octane gasoline, even when I have no 'check engine light' indicating any problems. Then what?

    If you regularly use gasoline which has a higher octane rating than is recommended for your particular engine, then you may be doing more harm than good.

    Too high an octane rating can cause the air/fuel mixture to prematurely ignite, before the spark. This can cause engine knocking that can lead to eventual damage.

    In my article I recommended high octane only to be used in one tankful at one time to clean out the catalytic converter. This is only to be done if your check engine light is on, in order to see if this 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 actually need your catalytic converter, or the sensor, replaced.

© 2017 Glenn Stok


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    • Glenn Stok profile imageAUTHOR

      Glenn Stok 

      8 days ago from Long Island, NY

      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.

    • profile image


      9 days ago

      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 profile imageAUTHOR

      Glenn Stok 

      9 days ago from Long Island, NY

      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.

    • profile image


      9 days ago

      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?

    • Glenn Stok profile imageAUTHOR

      Glenn Stok 

      3 weeks ago from Long Island, NY

      Erik, The fact is that my car is 20 years old and the method of using high octane gas has helped my avoid getting a new catalytic converter. Say what you want, but my experience, and that of the those who tried my method, speaks otherwise.

      I'm sure it doesn’t help is all cases, but it has worked so far. My light came on a few times in the past ten years with a P0420 code, and this method helped keep it clear for a couple of years each time.

    • profile image


      3 weeks ago

      This sounds like a sales pitch for a code reader. How would this method work for cars that are already running premium gas?

      When catalytic converters first hit the threshold of setting a catalyst efficiency code, the codes can be cleared and it will often take a long time before the code resets. This isnt because you "cleaned it out" with overpriced gasoline. It stays off because the cat wasnt that bad yet. Catalytic converters should be properly diagnosed using proper methods which involves more than a code reader.

      Also, catalytic converters are only designed to last about 10 years. 10 years is a lot longer than the average american keeps a new car. At 10 years, the cat can just be worn out.

      99% of the time a P0420 sets, the catalytic converter is the problem.

      Sorry, Glenn. Your "technical background" does not make you an expert in catalytic converter diagnosis.

    • Glenn Stok profile imageAUTHOR

      Glenn Stok 

      3 months ago from Long Island, NY

      Chris, since you already have a new catalytic converter installed, then the method I described will not work. It’s already a clean unit, assuming the shop put a new one in.

      Since your check engine light came back on, that means it was not the catalytic converter causing the error. Do you know the error code the machanic read? It’s useful to have your own code scanner such as the one I mentioned.

    • profile image


      3 months ago

      Just had my cat replaced and engine light came back on, can I still use your method to fix it?

    • Glenn Stok profile imageAUTHOR

      Glenn Stok 

      3 months ago from Long Island, NY

      Jeff Barge - All auto repair shops have diagnostic scanners. If your machanic is just guessing or not telling you the code, you’re going to the wrong one.

    • profile image

      Jeff Barge 

      3 months ago

      Why doesn't the law require auto repairers to buy this device instead of makng expensive replacemenets?

    • profile image


      11 months ago

      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.

    • profile image


      12 months ago

      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 profile imageAUTHOR

      Glenn Stok 

      12 months ago from Long Island, NY

      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.

    • profile image


      12 months ago

      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 profile imageAUTHOR

      Glenn Stok 

      12 months ago from Long Island, NY

      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.

    • profile image


      12 months ago

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

    • Nell Rose profile image

      Nell Rose 

      14 months ago from England

      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 profile imageAUTHOR

      Glenn Stok 

      14 months ago from Long Island, NY

      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 profile image

      Nell Rose 

      14 months ago from England

      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.

    • MsDora profile image

      Dora Weithers 

      14 months ago from The Caribbean

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

    • FlourishAnyway profile image


      14 months ago from USA

      Extremely useful information, Glenn! Thank you!


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