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P0420, getting to O2 sensor on an AWD

10173 Views 13 Replies 6 Participants Last post by  mec
I have been getting a p0420 code this month, and after lots of reading it maybe an o2 sensor (bank 1 sensor 1). I went under the van yesterday but could not put my hand on it. On an AWD do you have to mark and pull the driveshaft? If so does the tranny oil leak out? The only other way I saw is to remove the Y pipe, but then I think I need new gaskets. But would have a visual up the CATS.
Any ideas...Colin ???
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I looked this up and (bank 1 sensor 1) does not determine the efficiency of the catalitic on that bank. Bank 1 sensor 1 is an air/fuel ratio sensor and is only feedback for fuel trim. I will try to dig up some more info on this.

Any exhaust leaks?


Thanks for the fast reply robo5, I have spent many hours searching here and whatever google pops up. One consensus is the o2 sensor and the other is the cat .
There are no exhaust leaks at all, as far as I can tell. We fill up at the same gas station all the time. It's a popular one with fresh gas all the time. (I hope)
At first the CEL and VSC came on and I reset it. then in about a week it came on again, I reset it. Then the next day it came on again I reset it the next day it is on again, and that's how it is now. The van has only 56k miles on it. And a very easy life so far, when I was underneath the van it looks like new.
I was thinking if I changed the o2 sensor and the CEL & VSC returned then it should the Cat and that should be under warranty. (if o2 sensor) I perfer to do it myself 'cause the closest Toyota dealership is either one hour north of me in Cranbrook BC, and the other is 2 hours south in Cord lane Idaho. So 20 to 30 dollars in gas and a day off work is a lot more than a o2 and the correct socket delivered to me.
Also when the CEL the cruise won't function also.
Thanks again Colin
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P0420 is the code directly related to the catalytic converter, or bank 1, sensor 2.

The sensors 1 (bank 1, sensor 1; and bank 2, sensor 1) are air/fuel ratio sensors...a different beast.

It's odd that you'd get codes for VSC, too; there's no relation between the systems involved, as far as I know.

At your mileage, get to your dealer with this code, to have things diagnosed and replaced under the emissions warranty! (If it's a catalytic converter, the repair would be well over $1000, and you may call your warranty into question if you change out your sensor yourself.)

Good luck with not getting the code anymore. It's a common issue on the previous generation of Sienna (1998-2003), and I learned from experience, unfortunately, well after my emissions warranty, of course.
Did you check your brake fluid level for the VSC issue first? Search for "VSC" on this forum for other tips.

Are you 120% sure it's the back sensor? The back one would be harder to get at. Did you crawl under the van and visually see the sensor?

The front sensor on the 1st gen was easy to change. Make sure the van was driven first so that the exhaust pipes are warm. I used a 23mm conbination wrench too remove mine.

I would try to disconnect the wire on the sensor first and then remove the sensor, by slipping the wire through the round end of the wrench and turning the sensor out. That's if you arm can fit in the opening.
Dealer would probably charge you close to $400 or even more just to change this sensor. I bought a bosch at Auto zone for $113.

Regards, Jason.
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The VSC light will come on when the CEL comes on in certain conditions. Yes check the brake fluid and if it is OK do not worry about the VSC light. It will reset once the CEL is turned off.

OK so we are dealing with Bank 1 Sensor 2 to find out if the cat is good or bad. Do you have any access to a scan tool with data? This is the best way by far. I have asked around and came up with this. Once the sienna is fully warmed up. Engine running at 2500 rpm in park. The rear O2 sensor (bank 1 sensor 2) should not fluctuate more than .5 volts. O2 sensors work of of 0-1 volt range. What you are looking for is the feedback of voltage from the sensor. Steady (or very little voltage change) means the cat is OK. If the voltage goes up and down ( +/- .5 volts) then the cat needs to be replaced.

Some of the older multimeters take an average voltage reading and you may not notice the voltage changing as it is too slow to read it so it can be misleading.

Now the above info is readings off a good O2 sensor and I would have to say at this point (educated guess) that the O2 sensor is good due the fact that it fluctuates in voltage more than .5 volts. If the sensor was dead it would have another code other than P0420.

Hope this helps out with which route you want to take. I hope all the best to finding out what is wrong. If any others can add please do so ;)
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My scan tool does do live data, so if it is not raining too hard tomorrow I will see if I can get voltage on the o2 sensors.

If all else fails we will take it in to a dealership
You guys are a great help and I hope someday I will be able to help others also
Thanks Robo5for the help,
I was able to hook -up scan tool on Saturday and read the voltages @2500 rpm
B1,S2...0 to 0.7v fluctuating

I ordered an O2 sensor for the rear bank to see if the sensor is acting up. If I still get poor readings I know the CAT is at fault. I am 99% sure I don't have exhaust leak etc.
The van has 57k miles on it and is only 6.5 years old so emissions warranty should still be good. I exported the van out of the US and imported it into Canada, I hope the warrenties are in effect.
UPdate.. I put the o2 sensor in about 2 weeks ago and no CEL yet. Everything seems fine ...
Thanks everyone for your input.
Have the readiness monitors completed their tests? You're not free and clear until the Catalyst Monitoring Readiness Monitor is "ready."
UHHH I don't want to sound dumb, but these are new words to me. Could you xplain a little deeper on readiness monitors.
Thanks Colin
The engine control unit (ECU) runs "tests" to make sure systems, including the emissions systems, are functioning properly. It only runs these tests under certain conditions, like when the vehicle is warmed up. Until the ECU runs these tests, the "readiness" status for that system is "not ready." If you can read OBD codes, like the P0402, you probably can read the readiness status.

Here are a couple of links. I don't have a good link for 2004 Sienna drive patterns, but you can get an idea of how readiness monitors work:

From Toyota/Lexus:

General OBD info from Connecticut:
UPdate.. I put the o2 sensor in about 2 weeks ago and no CEL yet. Everything seems fine ...
Thanks everyone for your input.
I'm just starting to deal with the P0420 code on my 2003 Sienna. Was wondering if the O2 sensor replacement of the B1S2 fixed the problem or if after the test completed you still got the code?

May I ask if you were by chance driving fast (85 mph +) when the sensor when off?
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