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1999 Es300 Cel P0420

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I had a CEL with P0420 (bank 1 cat below efficiency) on for the last few months. I tried to do some research here but couldn't find any definitive answer. Most say it's either the cat or the O2 sensor. I finally got around to do some diagnostic with the OBD-USB tool and got the following waveform:


It appears that Bank1 Sensor2 is oscillating too much. One may conclude that the cat is bad. My questions are:

1) Is it true that the car (99 ES) has 3 catalytic converters (1 for each bank of V6, and both feed into a bigger cat) and 3 sensors (one before bank1 cat, one before bank2 cat, and one after the main cat)?

2) If the main cat is bad, why is only the code P0420 on instead of both P0420 and P0430 being on?

3) Is there a way to tell if it's the cats for either bank that's bad?

I truly appreciate the input or insight that you may have.

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you raise some interesting questions but first tell us how many miles you have on this 99 and if you ever had the cat(s) or the o2 sensors replaced. Also which engine is it eg 1mz-fe) The reason is this may help about how to proceed.

usually it's the "sensor 1" o2 sensor that goes bad, not the one on the cat, but the code says you may have a problem w/ the cat on bank one.

looking 0420 up, the trouble areas indicated are reportedly 1) gas leakage on exhaust system, 2)air fuel sensor bank 1,2 sensor 1) 3) heated o2 sensor bank 1, sensor 2, 4) 3 way catalytic converter.

This suggests that you actually have AF sensors as the first "O2" detectors, the ones in the manifolds (the AF sensors are a newer version of the old 02 sensor), but the sniffer downstream is an older style O2 sensor.

I like to think about what is the easiest and/or least expensive step first, then proceed with the next etc. I'd first do a visual/audio inspection of the down pipes -- either crawl underneath or take it to a muffler shop (easier and might be free...). If that checks out ok, I'd next replace the AF and/or 02 sensors ($50-$80 for the latter at Rock Auto). If that doesn't work you'll have to reckon with the cat. The bad input to the computer, shouldn't effect the way the ECM manages the engine, it's just a warning. But you may have trouble getting an inspection in many states. Sometimes banging on the cat w/ a hammer breaks up the matrix inside leading to better performance. Of course the ecological thing to do is just replace the cat, if the replaced 02 sensor doesn't fix it. I used to fret about replacing the 02 sensor if that wasn't the problem, but you have to think of them like spark plugs, they go bad after so many miles (80k?) and don't work as well when they get old.

BTW, those are pretty cool graphs. what obdII program are you using (mine doesn't do that...)? Was the graph taken at idle? If so it appears bank 1 and 2 sensor 1's are flatlining, and I'm not sure if that's appropriate; it may be normal if these are AF sensors. The up and down for the o2 sensor seems about right. However this makes it hard to explain why the code is being thrown.....

Keep us posted on your progress.



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Hi Lexis:

Thank you for your detailed reply. I guess I forgot to mention some basic info about the car. It has 163K miles, which is about 90% highway for the last 80K miles. I believe the engine is the 1MZ-FE3, which is different from 97-98 and has more in common with 00-01. The sensors and the cats were never replaced. The CEL would come on and off about a year ago, but now it just stays on all the time. I bought the OBD-USB cable and installed a software in order to figure out exactly what made the computer throw the code. The software is called OBD 2007, found at It's actually not a free software, but it does provide a free 7-day trial period. I would hate to replace the O2 sensor if it is functioning properly, especially because B1S1 is sandwiched between the engine and the firewall and is kind of hard to get to. The graphs were obtained while the engine was idling between 750-2500rpm. I'll take it out on the road in the next day or two to see if it does anything different. Looking at the graphs, I would think that the B1S2 shouldn't be oscillating like that if it is downstream of the cat. I'll also try to erase the code first just to see at which point the light would come on again (I am sure it will come on again).

Once again, thank you for your valuable input.

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  • 6 months later...

I forgot to mention one more thing. The fuel economy remains great even with the CEL light on. With or without the CEL, it's getting about 27mpg in the winter, and about 29mpg in the summer.

I have 99 ES300 150K and got the same code P0420 and only 18 mpg. What did you do to bring it up 27mpg.

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