P0420, getting to O2 sensor on an AWD

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 ...so 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

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.

http://www.princessauto.com/hand-tools/wrenches/combination/8068132-23-mm-combination-wrench

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.

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