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mathman32 12-19-2012 06:27 AM

Catalytic Converter?
I have a 2005 Honda Accord LX (4 cylinder). Over the summer, my check engine light came on reading PO139 and PO420. This is either the downstream O2 sensor or catalytic converter. I changed the O2 sensor and the check engine light went away. Now, both codes are back and I am afraid it is the catalytic converter. Is there a way for me to tell which one it is? I was thinking about taking it to the dealership and having them run a test on it so I don't buy an expensive part and that not be the problem. Any help on this??

ddawg16 12-19-2012 07:21 AM

Cat's have a finite life....I would say that if your over 100K miles....then it's most likely time to replace the cat.

What is your mileage?

joecaption 12-19-2012 07:54 AM

I would not take it to the dealer and have them change it.
Fine to have them test it, but there going to charge you at least double what a reguler garage of muffler shop would charge.
Here's an idea on what just the part would cost. L 4 Cyl

It's a simple change out, it just bolts in place.

Mort 12-19-2012 08:01 AM

It's possible the O2 sensor is bad again. Cats only go bad if the car isn't running properly, ie if its running rich.

mathman32 12-19-2012 09:27 AM

I have about 147,000 miles on it and I don't think it is the o2 sensor again. I put a factory sensor on it so I hope that's not the problem. I can do the job myself, but I want to make sure it's the cat first.

Chokingdogs 12-19-2012 03:42 PM

Odds of another O2 sensor being half of one percent.

Odds of a 7 year old cat, with 150K miles on it going south.....99.5%

There's no real way to "test" a cat, the downstream O2 sensor(s) essentially do that, since they're there to measure the cat's efficiency. If the cat is bad, and produces emissions out of scope, the post-cat sensors read that and throw a code ( check engine light ). One can "test" a cat with one of those infrared temperature readers.....since a cat during normal operation will be at a higher temp than the exhaust going in, a temp reading on the cat significantly higher or lower than incoming exhaust could mean it's bad. That's not foolproof though, as an improper mixture would cause that as well, either too rich or too lean......BUT.....if that is happening, odds are you'd be getting a code of something else being amiss as well.....pre-cat ( if there ) O2 sensors, MAF, cam position, intake air temp sensor, etc. etc.....

D-rock 12-19-2012 05:45 PM

Those year four cylinder Accords are known for oxygen sensors going bad, but if you installed the correct OE oxygen sensor then it most likely is fine. The cat conv after 150,000 miles is most likely the culprit.

Did you install the sensor your self? I only ask that because some times the threads will get damaged during removal and the new sensor may not seat fully during install. If any air gets by then it could cause some inaccurate codes.

mathman32 12-19-2012 06:28 PM

I installed it myself. It sounds like its the cat and what you all are saying makes sense.

Thanks people

ddawg16 12-19-2012 07:14 PM

Here is a good link for some good information...

Basically....if the engine is running should get 120K+ miles out of a, they do not last forever....and if you use any 'fuel treatments', you reduce the life of the cat.

ukrkoz 12-19-2012 07:39 PM

PO420 Catalyst System Efficiency Below Threshold (Bank 1)

What this simply means is that your catalytic converter is not working properly. There are a number of things that could be causing this p0420 catalyst efficiency code. The most common thing is the catalytic converter itself is no longer functioning properly. The other likely thing is the rear O2 (oxygen) sensor is no longer working properly. Other things could include exhaust leaks, damaged exhaust pipes, damaged O2 sensor wiring/connectors, plugged catalytic converter, etc. The easiest thing to do first is a visual check of things. Visually inspect the exhaust system for leaks, check the catalytic converter for dents, holes, severe discoloration, and check for a rattle inside. If any of those symptoms are there, the converter likely needs replacement. Then, visually inspect the downstream O2 sensor (behind the converter). Check for broken wires, obvious faults, etc. If all that checks out, you'll want to check the operation of the O2 sensor. To do that, you'll need access to a scan tool or oscilloscope. Check that the waveform is pretty steady. If the reading fluctuates then the sensor is likely bad and will need to be replaced.
On some vehicles such as some Subaru models an ECM reflash will fix the problem. As you can see a P0420 can be caused by many things, so if you have any doubts as to whether you can fix the problem yourself, please seek the advice of a professional technician. Good luck!

msmith5554 12-26-2012 09:19 PM

I have been a Honda tech for over 15 years. The cat has gone bad. If you plan on keeping the car, spend the money and purchase a cat from the dealer. If you buy aftermarket you will be doing it again sooner than later.

r0ckstarr 01-08-2013 08:22 AM


Originally Posted by msmith5554 (Post 1080283)
If you buy aftermarket you will be doing it again sooner than later.

I agree. I have less than 100k miles on my aftermarket cat, and am about to have it replaced. When idling, it sounds like marbles bouncing around inside of my cat. The insides have come apart and are vibrating around at idle.

Mine isn't a Honda, but all cats pretty much work the same.

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