Toyota - Pesky PO420 DTC

[Tim Martin]

Does anybody out there have a good way of diagnosing this trouble code? Is there a good way of determining whether the problem is the downstream O2 Sensor or the cat? I have so often followed the diagnostics given in a troubleshooting flow chart only to wind up replacing both the sensor and the cat. I got a 01 Toyota Sienna in here right now with the same issue. These cats are costly on this one.

[ricmorin]

Monitor the upstream and downstream O2's on a scope. See if the post O2 is following the pre. If it is, the cat is probably bad. As you know, that post sensor should be steady after the cat lights off. Toyota is usually good about identify slow O2's and bad heaters. I use a generic laptop scanner for this so all I have to do is plug into the OBD port.

[brianp87]

I also see how they are switching. I do find alot of bad aftermarket cats.

[Pauls Automotive]

I do the same. I always to to go OEM when replacing cats. Too many problems otherwise

[crmeyers]

Like Ric stated in the above post, if the downstream O2 readings mirror the upstream, you've probably got a bad cat. I attached an example of a normal reading and an abnormal reading. And yes, stay away from aftermarket converters on these. They will bite you in the a@&!

[Tim Martin]

Understand the lab scope readings. So what is being said is that if the sensor can generate that kind of signal then the cat is at fault. Right?

[brianp87]

The sensor after the cat shouldnt read the same as the front sensor if it does then the cat isnt working.

[steven kiser]

i hate to say it but on a toyota with these symptoms i'd bet a meal that it needs a cat.

[crmeyers]

I'm right there with you Steven. Where we eatin' at?

[steven kiser]

red bones........or adams ribs.........mmmmmmmmmm

[jbadenoch]

tune-up parts cure 80 percent of my cat issues

[a&mauto]

Tim, I agree with everything said so far. Also, if there are any other codes, fix them first. Make sure it is going into closed loop. If its not, the cat will not light off. I also compare inlet temp to outlet temp after running it at 2000 rpm's for a couple of minutes. Outlet should be at least 100 degrees hotter. If its the same or cooler, and its in closed loop, and your scope pattern is like the lower one, you probably have a bad cat.

[steven kiser]

many vehicles with that mileage do need tune ups. if the scope patterns are mirroring each other a tune won't help. i was at a seminar a few weeks ago and there were booths. at the exhaust booth they were pushing a cat flush. any thoughts on this?

[brianp87]

Ive never heard of it what is it.

[Tim Martin]

Now steve, you pullin our leg or what? What's a cat flush? Ain't not heard a that yet.

[wbuxton]

It's illeagle to flush any animal in Texas. Except a goldfish.

[steven kiser]

i've seen this stuff on the shelves of the part store. i'll look into it and post my findings. i'm a firm believer that chemicals like this are a snake oil repiar. like in the movie "the out law josie whales" when he spits chaw juice on the sales man and askes "can it get out tobacco juice"

[crmeyers]

Found the cat flush you're talking about:
http://www.youtube.com/watch?v=WofFb_eO ... dded#at=28

[Tim Martin]

Is that cool or is that cool?????

[Goose]

That is a cool cat flush. Wait till they get the water bill!