Catalytic Converter: Avoid Unnecessary Repairs and Save Your Money
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
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.
This article is accurate and true to the best of the author’s knowledge. Content is for informational or entertainment purposes only and does not substitute for personal counsel or professional advice in business, financial, legal, or technical matters.
Questions & Answers
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.Helpful 23
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.Helpful 22
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: https://amzn.to/2M3muA5Helpful 5
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 indicative of catalytic converter issue. Is it too late to try the high octane gas method you mentioned?
The rattle you mentioned indicates that something might be loose. That might also be a result of a badly damaged catalytic converter that definitely needs to be replaced. So you should have your mechanic put your car on the lift and eyeball the situation.
If you’re lucky, something else is rattling. If that’s the case, then I would say you should try the high octane fix, especially since you say your check engine light has been on and off for two years. That would indicate that the unit just needs a good cleaning that a tank full of high octane would do. But first get that rattle checked out.Helpful 10
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.Helpful 10
© 2017 Glenn Stok