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Another P0420 post

345 Views 6 Replies 3 Participants Last post by  jackwong817
I looked briefly over previous posts on this topic but I could not figure out if it helps with my situation.
So I have a 2005 Scion TC that I really like as it gave me no problems with respect to reliability and driving. It has 150K miles on it
However, the car is due in a couple of months for an emission test.

About 5 years ago I started to have P0420 popping up. I have a Scangauge II installed that I keep it connected all the time so I cleared the code several times but they came back usually after a week or two depending on how long I drove.
No symptoms otherwise as far as I can tell...I decided to replace both oxygen sensors. First the upstream one and that did not do anything. Then I changed the downstream one and that fixed the problem for about 3 years.

A year ago, I had the same code coming up again. I changed again the downstream O2 sensor and the code again disappeared for about one year until recently. Now the code came back again. This time I looked more carefully and I realized that the O2 sensors I used did not look exactly like the original although they were compatible according to PartsGeek. I decided this time to order one from Toyota.

This time, the code did not dissappear. Usually all is fine while city driving. It comes on after driving at high speeds on the highway for a certain amount of time. Obviously I will have to go to the mechanic and maybe pony up the money for a new Cat. However, during these years, I did not notice any symptom at all...If anything the car runs quite smooth for its age. Furthemore, I have this feeling that there might be something else going on given the fact that changing those O2 sensors solved the issue for quite a long time

Is there anything else I could check? Could this be purely a software issue...I did replace the batter 3 months ago and I think the code came back after that. Not sure if its related. Is it worth trying to replace the ECU? I heard about some software issues for these early models.
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Sounds like the cat in the header is bad. You might be able to find a cheap one from a scrap yard or eBay.
Thanks. Do you know if one can replace just the CAT on a 2005 Scion TC or do you need to replace the entire exhaust manifold.
Thanks for the reply. I am still not 100% convinced it is actually not some electrical/software issue. Changed O2 sensor (downstream) with another brand (Denso I think) and the code disappeared. I know there is no obstruction in the converter (a mechanic supposedly checked for that). The mechanic said that maybe the catalyst is simply spent which is the reason why it does not affect driveability. We will see how long it will take until the code comes back again (last time it took a year and half).
Here's what I know for sure in the last 4 years since this issue began

1. The original Toyota brand downstream O2 sensor triggers the P0420 code immediately after the monitor is ready
2. The Denso brand does not. First time it took 2 years for the code to re-appear and then second time it took 1 year

Let's say something happens that makes these O2 sensor burn out. But my understanding is that , in that case, I will get a different code not 0420. If indeed the efficiency of the catalytic converter is lower than the threshold why doesn't the code appear immediately (with any sensor) rather than later?

At some point I will replace the converter but I just ambition myself to understand exactly what goes on before doing that
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Also, just to add more fun to this. After replacing the O2 sensor and making sure the code did not appear again, I also checked for the readiness of the monitors (I have a Scangauge connected at all times to do this). All were ready except the Evap monitor. This is the only monitor allowed to be incomplete for emission test purposes. Went in and passed the emission test (which in my state is just them hooking up to the car computer and retrieving the data). After I passed the test, I noticed that now both the Evap and the Catalyst monitors were again incomplete.

After driving 20-30 miles the Evap monitor is complete but the catalyst is still incomplete. So now I wonder if these machines that hook into car computer can actually cause some software problems. Makes me wonder even if my own scangauge that I keep connected all the time might do something. Either way, I will wait and see but I heard on another forum that the ECU on old Scions needs a software update or something like that.
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