P0420 Code - Engine light glitches on/off
 

Hi! I need advice or answers lol, whichever. I have a 2015 FRS and I recently changed my headers. After a week it started throwing the P0420 code. And I’ve heard so many ways to fix it. Get it tuned, O2 needs an extension or replaced completely.

The thing is.. my engine light cuts on and it’ll stay for a while and than cut off. It’s been off for almost a week and it came back on 2 days ago. I work at a shop, but we’ve been super busy and I haven’t had the chance to pull it in and have it checked.


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You are not getting all the benefit of the header without a tune so just get a tune. Those O2 extensions are hit or miss anyway.

I'm going to assume you bought a header with a replacement cat. That's where your issue is.

The OEM primary cat has 1000 cells of catalyst per inch. The replacement in all the catted headers is usually a 200 cell count. That's a major reduction in overall catalyst efficiency.

So what's the tune about? When the ECU measures efficiency, there's usually some tolerance for things going a little out of range in an attempt to smooth outliers from the samples. This is usually a moving average of the latest X samples. All that means is if you're going in and out of expected ranges (Say in range driving conservatively, and out when you give it the beans) it can throw a code and then determine it to not be occurring later on. What your tune will do is disable that code so your CEL won't come on (Plus all the little tweaks to make power ;))

The answer is to get a tune. You can try the cheap fixes like an extender but we're not in the 90s so they're not 100% effective 100% of the time (Much more like 50/50, depends on how you drive). A replacement sensor makes no sense at all since you're getting the expected results of decreasing catalyst efficiency in exchange for reduced backpressure in the header.