I replaced my replaced Catalytic Converter and after about six months, the check engine light went on. I went to get it checked and it says the Catalytic Converter is bad.
I talked to a guy from the manufacturer and he was a mechanic for over 30 years and he said there could be more going on than just the cat and if I want that one replaced, I would need to do a full diagnostic. Another friend, who was a mechanic, said the same thing. He also said, that when the cat goes bad, the O2 sensor goes bad as well. Another friend, who is a mechanic and has worked on my car, says it is only the cat that’s bad.
Who’s right?
No real way to tell who is right. The converter could be bad. The oxygen sensor may be bad and the cat is ok. If you posted the numeric code that the computer is storing someone might be able to help.
I disagree with the guy who said that the 02 sensor goes bad when the cat goes bad.
The full diagnostic that they’re recommending is the way to go.
In the ether of the internet, they may all be right to some degree.
The code is simply the ECU reading sensors and deciding that what they are reading is consistent with a bad cat. It may not actually BE bad. Other things must be checked to determine the actual problem. The mechanics are making a professional assessment based on their experience not actual diagnostics of your car.
Considering 6 months after a cat was replaced the code is set yet again tells me the underlying cause of the cat failure wasn’t fixed.
But you didn’t tell us ANYthing about your car. I don’t even know for sure if this question is about a Toyota Corolla or not. You don’t say. If you’ve shared this little with your mechanic friends, no wonder you got some variation in answers.
1st, the code that comes up is 0420 - Catalytic converter.
2nd, prior to replacing the first cat, my friend who worked on the car, coded it and fixed a lot of other issues and then went to the shop and found out that the cat was bad. My friend replaced the 1st cat and a year later, the check engine came up which said the cat was bad, so I got a replacement. 6 months later, the code came up again.
Everyone’s wrong. The code did not come back as “bad catalytic converter.” It came back as “catalyst efficiency below threshold.” That could be a bad cat. It could be a bad oxygen sensor. It could even be a hole in the exhaust.
Before people start hurling parts at the problem, they need to do a proper diagnosis to figure out what the problem is. The problem with modern cars is that their error codes are meant to give you a nudge in the right direction toward figuring out what’s wrong. But many people, pros and amateurs alike, think the error codes are the automotive equivalent of a prophet and assume that whatever parts are mentioned in the code are the parts that absolutely need replacing. That misconception leads to a lot of needless expense and wasted time.
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That “misconception,” is responsible for making a lot of boat payments 
.
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Thank you!
There have been THOUSANDS AN THOUSANDS of perfectly good catalytic converters replaced over the years, when the real problem was actually a faulty O2 sensor.
You need further diagnosis. A good mechanic would have done that.
A Toyota experienced mechanic should compare the pre-cat and post-cat O2 sensor volt vs time waveforms. That’s the best way to determine if the cat is the problem, or something else is. For example it might be possible on your Corolla when replacing the cat to inadvertently switch the wiring harness connectors for the pre-cat sensor and post-cat sensor. That would likely cause various symptoms, including damaging the replacement cat, and the cause might not be obvious to an untrained eye.
Here’s one discussion, with several possible causes:
"If you do a search on this forum, the P0420 CEL code comes up quite a bit for an 8th generation Corolla (1998-2002) and some of the 9th gen Corollas (2003-2008). In most of those cases, the catalytic converter is perfectly fine - it is usually an exhaust leak, bad downstream O2 sensor (lazy sensor), or something else that is causing the CEL to come on. There is no sense in swapping out a decent, OEM + expensive catalytic converter, for a so-so universal one - until you completely verify that it is indeed a bad catalytic converter. With your mileage and age of car - I would suspect a degraded downstream O2 sensor or a bad exhaust donut that is causing the CEL instead of a bad catalytic converter (verify with a sniffer or tailpipe test). "
The first cat was diagnosed at a shop as bad. The second and third was done by a friend who has a gadget that talks to the car’s computer and it read cat system efficiency below.
Because of many people’s second and third and fourth, etc, opinions, I’m getting it checked tomorrow.
Are you having to add oil to this car? How often and how much?
Are you using any gas or oil additives?
Not knowing where you are, is this a gas engine ?
Yes it uses gas. Don’t need to add oil very often. Once in a while, I do. About a quart. No additives
1st, the code that comes up is 0420 - Catalytic converter.
2nd, prior to replacing the first cat, my friend who worked on the car, coded it and fixed a lot of other issues and then went to the shop and found out that the cat was bad. My friend replaced the 1st cat and a year later, the check engine came up which said the cat was bad, so I got a replacement. 6 months later, the code came up again.
P0420 can mean the catalytic converter is bad, but often it’s not. I had (I think) P0420 and P0421 or 22 awhile ago on my Focus…after looking at the waveforms, we determined that it was a bad O2 sensor, not the catalytic converter. Replaced the O2 sensor and I haven’t seen the CEL since (@Tester had a good picture of the waveforms showing the difference between good O2 sensors vs bad ones, any chance you could throw that back up?)
P0420 is not neccesarily a bad cat. That is exactly the fallacy I tried to tell you in my post. These are the posibilities that causes a P0420 code;
Possible causes
Faulty Three-way Catalyst Converter Bank 1
Exhaust Tube Intake Air Leaks
Faulty Oxygen (O2) Sensor
Faulty Fuel injector(s)
Leaking Fuel Injector (s)
Faulty Spark plugs
Improper Ignition Timing
Faulty Engine Control Module (ECM)
Dirty Air Filter
Read more: https://www.autocodes.com/p0420.html
That code just says the O2 sensor constantly sends signals to the ECU of it’s measurements. When those numbers are not in desired range then a code is issued. These codes are there to help make a proper diagnosis. A good mechanic will then have to determine if it’s the O2 sensor that’s faulty or there is an actual problem with the Cat.
Many unscrupulous mechanics will jump to replacing the Cat (and O2 sensors) because there’s a lot more money in replacing a Cat then just an O2 sensor.
Very unscrupulous mechanics will replace the O2 sensor, yet charge you for replacing the Cat. Firestone and Sears were caught by investigative reporters doing this a few years ago.