P0420 -- testing o2s and MAF and more?

[murkyturtle]

I have a 2001 Sienna XLE with 69k and the check engine light on. My problem is similar to SiennaDriver in "CEL codes morphing, confused."

The first check with a scanner indicated a P1135 --the pain in the rear A/F sensor heater circuit. I had to wait a couple of months for the snow to go away so I could crawl under there to replace it. That done. The scanner then indicates a P1155-- Sweet! After the other one, this one you can replace in your sleep.
That done, now the scanner has given me a P0420--CAT efficiency code. UGH!

It seems from searching around here that the P0420 is somewhat of a controversial code. I don't want to throw a CAT at this vehicle if I can find a cheaper part to make the problem go away.

My questions are:
Why all of a sudden the P0420? Would having a P1135 or P1155 prevent that from coming up earlier?

There's not a lot of talk about the actual O2S around here: even though the live data indicates it is functioning, since the other two went bad, is it a good assumption that the O2S is weak or going itself? What is the best test for this?

There is some talk of the MAF perhaps not working properly that can set this code. Is there a test you can do on the MAF with a DVOM that will indicate its worthiness?

Are there other components contributing to this code; how do you test them?

Also, is there a place where I can check to see if my particular vehicle is a candidate for ECM flash update?

CATs don't just go bad. How can the CAT have gone bad on a vehicle that has been maintained and otherwise runs well? It just doesn't add up. Your thoughts on this would be appreciated.

[chuck's sienna]

You could try replacing the O2 sensor just after the cat and go from there. Just so you know, I've had a PO420 for three years and got tired of seeing the check engine light glowing at me all the time so I put a small piece of electrical tape over it. I,m at 249K miles now too, still runs great. Did a hwy gas mileage test last month, got 28.25 mpg. That proved to me everything is fine internally. Hope this helps.

[Black Sunshine]

First, have you tried just clearing the code? I've gotten that code twice in 150K miles and after clearing it, it didn't come back.

The sensor can be checked with a multi-meter. I don't know the specs off hand.

[SiennaDriver]

SiennaDriver here, having a similar situation.

My experience is not on the same level as some of the expertise you'll find posted here, but FWIW first I determined the heater wasn't working in the front sensor. Replaced that with a Denso brand, then the rear sensor code briefly came on. I reset the CEL and a short time later the P0420 came on. I reset that, it came back once briefly and went out - and hasn't returned. My totally uneducated guess is that the car has to adapt to the new sensor and throws codes in it's confusion. Enjoying a code free car (knock on wood).

I don't know if it's related, but I ran a jug of the Techron (sp?) fuel injection cleaner that Costco sells shortly before the P0420 vanished.

I'd like to know if there's a way to monitor the sensor after the converter while in use, not just check it, with a VOM or if specialized equipment is really necessary.

And if someone can comment on the last picture in my post, with the muffler paste and the effect it might have on a converter, it'd be appreciated.

[Black Sunshine]

I think that you can check the voltage levels while it's running. I think that higher end scanning tools can monitor it while it's running. They might qualify as specialized equipment.

Are you saying that the muffler paste is hard like glass? I think that there is just as much of that stuff on the inside as the outside. Regardless of what texture it is, it probably not good for the cat. Cats getting clogged is about the only thing that can go wrong with them. I also think that the paste stuff probably blew right out of the fresh connection and went on down stream to wherever it went. In this case the cat. I would hope not though. Cats are expensive. Exhaust isn't fun to work on.

Out of a lot of vehicles with high mileage I've only had one bad cat. It was clogging because the insides of it were broken up from impacts that the cat had suffered. On that vehicle the cat wasn't being monitored by the system but the cat was causing a loss of power.

[murkyturtle]

Clear the code and cross my fingers, eh? That would be the least expensive fix...

Throwing in some "mechanic in a can" isn't a bad idea either...

My scanner shows live data and that the after CAT O2S is functioning, but considering, I think, that this code is thrown by comparing the before and after sensors; I was wondering if maybe the after Cat sensor may be weak...

BTW The electrical tape idea is stellar--don't think it will fool the emission testing team though.

[Black Sunshine]

No the tape won't fly when you get it tested.

I wish mine would get 28MPG. We topped off our tank today and a lot of the miles on it were highway and it was 21 and some change. Pure highway on flats it will only get 24MPG. Today has been all highway but we were in the mountains on steep steady grades.

[murkyturtle]

@SiennaDriver: You can monitor the O2S while in use with a DVOM. O2 sensors create electrical feedback by the gases that are flowing through them. So technically you could run the car (it will have to be warm--the Os sensor won't have it's heater to help it) with the sensor unplugged with a DVOM connected to the output wires to see what it's doing.

Which leads to my original question (It sucks only knowing part of a testing procedure): what exactly should it be doing (volt range) and how often? How do you know it's bad or weak?

[murkyturtle]

Well, I cleared the code, sent some Seafoam through the engine at the throttle body, then drove it a bit to give the ECU time to check the systems. The code is back--it seems it has to happen twice to turn on the MIL-- it only set once so far, so the MIL is still off, but that won't help me at the testing station.

Curiously, the after CAT O2S is now reading a bit lower than it was before the Seafoam. The before CAT sensors have only changed by .1 v (out of a potential 5 volts I believe)...so the ECU doesn't seem to be relying strictly on that O2S to calculate the mix.

Incidentally, I found an informative website about O2 sensors: http://www.engine-light-help.com/oxygen-sensor.html

My next step will be to replace the after CAT O2S. Other suggestions are welcome.

[chuck's sienna]

Yes here in more miserable every day Maryland (gov't interference) we have emission testing. I recently passed only because I ran fuel system cleaner thru my van and the same day turned off the CEL with my small scan tool. The light stayed off for three days, and during that time is when I took it in. The very next day, the light came back on!. The next emissions testing notice won't come for two more years, so I am good to go.

[murkyturtle]

Just wanted to give you an update. I replaced the after CAT O2 sensor and reset the MIL. After driving a bit, the MIL has stayed off; the scanner shows that the system has checked out the O2S and CAT--and no codes.
Incidentally, in the live data report, the new O2S seems to have more steady/consistent readings (never fluctuating more than .02 v or so). The old one, although still functioning, fluctuated more wildly than that.
Now I just have to wait for the Evap to come on line to test it at the emissions station. Any ideas of how long that will take?

[chuck's sienna]

Three complete driving cycles, that is, three trips where the engine becomes fully warmed up and completely cools off afterwards.

[murkyturtle]

Thanks Chuck. Evap came on-line, no codes, no light. Took it to emissions and passed. I'm free for another two years.

[Black Sunshine]