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P0420 Error Code- 1999 Toyota Avalon XLS

7K views 4 replies 3 participants last post by  BamZipPow 
#1 · (Edited)
I'm getting a P0420 error on my car again.
I own a 1999 Toyota Avalon XLS V6 3.0L with 193,800+ miles on it.
I have recently already had my catalytic converter replaced May of this year, 2017 ($523) as well as the O2 Sensor (Bank 1, Sensor 1). As of today, I just had the ignition coil packs replaced (3 total) due to cylinder misfires (cylinders 2, 5, and 6- P0302, P0305, P0306 respectively). Those codes are gone, but now I'm getting the P0420 code again. Any ideas on what may be causing this to pop up again? I really don't have the money to take it to the shop for another replacement. Should I check the mass airflow sensor?

Thank you,
COnochie

-I have reset the computer and my check engine light is off again. I will provide updates as they come.
 
#2 ·
Sounds like the wrong cat may have been replaced...if your engine is anything like my CA-spec ones, it'll have three cats. You could try sticking an O2 sensor spacer on the downstream O2 sensor. That'll definitely get rid of it, but it will fail visual when you go to smog (if your state does that).

I was just looking for VVT valves for my ES300 on Amazon, and one reviewer said that they had a pesky P0420 that disappeared with a new valve. Go figure, I've never seen that kind of intermittent VVT valve failure on the 1MZ, they usually seem to get sludged up and/or burn out. Then you get the running-rough-when-warm phenomenon.
 
#4 ·
Yeah I bet the wrong cat was replaced, or more than one went bad. There's a member in the gen3/4 forums who's had good luck with Cataclean. You could also try to run something like TC-W3 for marine engines to drastically reduce emissions before a smog check.

Keep in mind that a shop normally won't install an O2 sensor for liability reasons, but you can for hobbyist purposes. Also of course, don't hold me or Toyota Nation responsible for your shenanigans 0:)
 
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