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Another P0420 post

342 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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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.
I had the same problem with the elusive P0420. I like you, did the least expensive item first which was the O2 sensor. But I also heard about Techron additive in the gas tank and driving with a full tank of Chevron gas. Again this did work but temporary. When my EVAP code came up it was my fuel cap, changed it and it was fine till four five years later. I applied a small amount of silicone PLUMBER grease around the rubber gasket of the gas cap and solved the EVAP code. The final check after many years was the front cat by using a laser temperature reader where the body of the cat, and pipe just after the weld should be a few hundred degrees different. if not a bad cat. Changed it and haven't had anymore issues.
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