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Hi everyone,

2016 FoST stage 3. I installed a CJ Pony Parts Catted DP and installed a Stratified tune with crackers and pops. I noticed a P0420 and immediately turned to the internet for answers. I got some answers online but in order to avoid the dealer, I am wondering if I can get some input from you people first. Anyone had P0420 before? Any solutions? Would the sensor piece that goes in the bung solve this issue?
 

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I recently purchased a used, 2 owner, 2016 ST3. It has been heavily modded with a catless downpipe, custom flowmaster exhaust, some type of K&N intake, cobb RMM, cobb shift knob, sound symposer delete, turbosmart bov....this is all i have found so far...

There was a P0420 code on it when i purchased it. I'm guessing the previous owner had a Cobb Access Port installed with a custom tune but the AP didn't come with the car. I just received a vibrant brand de-fowler from amazon to put on the 2nd O2 sensor to see if that will make the code go away.

I had a 2015 Honda Accord sport MT with a catless downpipe that threw the same code without the defowler that came with the downpipe. I put the defowler on and it went away.

There are multiple people on the forums saying that a de-fowler/non-fowler will make the code away. I will report back after monday and let you know if that worked.
 

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Update: I installed the vibrant J style defouler on the Downpipe today and the P0420 code is gone and hasn't came back after 100 miles. All my readiness monitors are good to go. I highly recommend it!
 

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Just ordered one be here by 1pm today Amazon is the best car is on jack stands waiting on the rain to stop to finish up with the new cobb catback / DP install. Lot easier to install this while the DP is on the bench. What Restrictor did you use I'm reading the middle one is most common.
 

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Drove about 40 miles tonight with the new setup I decided to use the largest restrictor since this is a catted downpipe. Haven't gotten any codes I think it worked I did notice little popping noise under high loads with more then 15 psi of turbo boost. Ill just keep my foot out of it maybe the computer will learn from it.
 
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