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Old 06-15-2019, 06:36 AM   #1
jtrouty3
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Member#: 408558
Join Date: Dec 2014
Default Possible Fix for p0420 on 2015 WRX

Hi all!

P0420...everyone has come across this code at some point in their journey. I will share with you how I fixed my p0420 code. I must disclaim, this may not fix your code but itís a cheap alternative and does not hurt your sensors to try this.

8 months ago I tripped a p0420. I scoured forums and was convinced I had a bad downstream oxygen sensor. The car had 90k miles on it so I figured why not spend the 100 dollars and pray to god it wasnít a bad catalytic converter. After replacing the downstream sensor the code tripped right back on. At this point, if I would clear the ECU the code would return within a drive cycle. Overall, I must have cleared the code over 20 times. With my tail between my legs I took it to my dealer. Their diagnosis was a bad catalytic converter. I was told the oxygen sensors were operating normally but the CC was dirty. I was quoted about 1k for parts and labor. I took it home refusing to get it fixed with a new determination to fix this issue. I pondered, how could this catalytic converter have gone bad? I kept very good care of my engine and regularly maintained my engine fluids and all of the maintenance recommendations. I purchased a borescope off amazon for 30 dollars so I could see for myself what the catalytic converter looked like. Upon inspection from both bungholes , the catalytic converted was not filled with grime or carbon build up. In fact, it looked perfect and clean. During the process, I pulled the upstream oxygen sensor. It was completely filthy. The vents on the outside of the sensor were even blocked with build up. I had an old bottle of mass air flower cleaner on hand so figured it would do the trick to clean gently without damaging. Trusting my gut, I avoided my urge to use break parts cleaner. I was so frustrated at this point, I was willing to sack the upstream sensor if it meant middle fingering the dealerís diagnosis. I screwed back in the upstream sensor and whoop dee friggin doo, code turned off and has not returned after thousands of miles.

Success! I hope this helps someone else. Sure would not hurt to try.
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Old 06-15-2019, 07:10 AM   #2
Crispy 1
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Member#: 478948
Join Date: Dec 2017
Chapter/Region: South East
Location: Hendersonville, NC
Vehicle:
02' WRX
SRP

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Where could one find this magical mass air flower cleaner? . Glad it worked for you. I may have try it on my beater Forester!
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Old 06-23-2019, 10:25 AM   #3
jtrouty3
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Member#: 408558
Join Date: Dec 2014
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Quote:
Originally Posted by Crispy 1 View Post
Where could one find this magical mass air flower cleaner? . Glad it worked for you. I may have try it on my beater Forester!


haha oopsie. 🤪
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