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.