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Big Russ

1996 Honda Accord P0420

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“Catalyst System Efficiency Below Threshold (Bank 1).”

 

What came first...  The chicken or the egg....

 

The front O2 Sensor is cycling as it should be

The rear O2 Sensor is steady (as it should be?)

https://photos.app.goo.gl/kmV5usT2v31fUYsE6

 

I havent checked both ends of the cat to see the temperature difference lately...  

 

Bad rear O2 Sensor?  Or bad kitty?

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No guesses?  I'm hoping its not the cat...

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it means the front and rear O2 sensors are reading too far apart from each other. could be lean or rich, could be ignition related, could just be a worn out cat.on a subaru, new plugs,air filter took care of it. one of the more difficult fault codes to make go away cheaply, because it can be caused from so many different things. a really good scanner is a big help with this one, then you can watch spark and fuel and all the emission stuff while its runnning to find whats out of spec...good luck. i would start with the basics if you havent already.

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Cat, and don't buy a cheap one! I had a GM product with the same code, replaced the cat, passed deq and the code came back 3mo later. I put the O2 sensors on a graph and started the car cold and the graph read like it should but when it got hot The graph shows the problem is the cat and not sensor. Replaced the cat with a higher quality unit and the code went away for good. When hot the front O2 sensor is continuously changing and the rear O2 Sensor evens out into almost a flat line. If the rear O2 looks like the front sensor when hot the cat is bad.

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Thanks Greg and Robert - there is a video link there of my scan tool looking at the data from both O2 sensors - the front O2 is continuously changing, while the rear is rock solid - hence why I was asking the question :).  Next time I look at it, I'll take my laser heat gun out there and see what the cat is doing - it should be hotter at the rear than the front if it's working correctly.  

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