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Catalytic Converter: Avoid Unnecessary Repairs and Save Your Money

As a technical writer with a science and engineering background, I solve technological problems to save money on car repairs.

View of Undercarriage of Car

View of Undercarriage of Car

Check Engine Light for Catalytic Converter?

Your check engine light is on. Your mechanic told you that you need a new catalytic converter, and it will cost you $900 (if you're lucky). 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 maybe you 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.

In this article, I will cover the following topics:

  • What is a catalytic converter?
  • What the PO420 diagnostic code means (and what you can do about it).
  • How to clean out your catalytic converter.
  • What to do if cleaning out your catalytic converter doesn't work.
  • Why every car owner should have a diagnostic scanner (and my preferred one).

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.

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 its 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.

Why Does the P0420 Code Keep Coming Back?

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

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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?

My OBD Diagnostic Scanner

My OBD Diagnostic Scanner

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 under $40. It works with all vehicles sold since 1996. Since then, all cars had to be OBD-II compliant.

How to Clean Out Your Catalytic Converter (4 Easy Steps)

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.

Step 1: Fill Up Your Tank With High-Octane Gasoline

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 the 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.

Step 2: 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?

Step 3: Shut Off the Check Engine Light With Your Scanner

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.

Step 4: Continue Driving and See If the Light Comes Back On

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.

Yes, This Method Really Does Work!

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.

Yes. 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.

Why You Should Own an OBD Diagnostic Scanner

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.

Once you have your own tool, similar to the diagnostic scanner that I bought on Amazon, you can always check your emission system’s status yourself. There is no need to pay a mechanic for a needless reading or get suckered into a repair job you may not need.

The way I look at it, getting your own 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: The P0420 sensor has gone on and off for about two years and 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 those issues?

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 Catalytic Converter. But since you removed it, it’s too late for that now. You need to have a new one installed.

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: I know that 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 think I should replace those three things?

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

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 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 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 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: 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

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