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Hi
I would want to know why I’m getting problems with my car. I just added a 3” exhaust with headers. Honda Civic si 2007. And had no 02 sensors just for a bit. I purchased the 2 oxygen sensors installed them and made the two codes go away. Now I have came to get a code p0420 after some bit of time. Idk what I can do? The o2 sensors are new and everything is where it should be attached. Will the car actually need the original o2 sensors denso. I have two different brand sensors but right out the box and not cut anywhere. Can someone please let me know what I can do?

 
Rep Power: 511

Did that exhaust you put remove the cat?

Sidenote, 3" exhaust on an N/A engine is quite excessive. I'd imagine you have a good amount of low end torque loss

 
Rep Power: 222

Did the exhaust include an aftermarket cat?

 
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The exhaust is a god snow full exhaust and DNA headers. I removed the old cat and muffler and got the full piping. I believe the aftermarket don’t really have a cat if you look at it by eye it’s more of bolder area from all the piping so i believe that is the cat. Doesn’t compare to the original tho. I’m no expert. I would say just have to beat the excessive sound cuz using the gas peddle in first gears the noice will be more noice then engine horse.p. At a glance wise. The torque is good. I would figure if I got, or getting. The 2 denso o2 sensors I wouldn’t have a problem even the car would run better and even quieter due to better parts. Right now it’s like having knock offs in my mind because of the trouble. And me not knowing.

 
Rep Power: 511

That's why you're getting a P0420 code, because there's either no catalytic converter or it's some non-OEM efficiency spec aftermarket one. I'd be willing to bet it's catless.

P0420 is "catalyst system under efficiency threshold"

You can either a) live with it or b) O2 sensor simulator (mechanical or otherwise).

 
Rep Power: 367

godsnow... ebay "vendor"
i see it is rated just as loud
loss or hp and torque where it matters most in a street car...
can't find specifics of CAT or dog being there or not

 
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That sounds kinda dumb saying it wouldn’t have a car when exhaust systems sold have everything as a fully assembled exhaust and not (not all parts included) and whatever you have to say about cat and dog I disagree hp gained for sure. I don’t need to ask you the question for the reason of telling right away. And the cat is just different not exact like what you are thinking. Obviously there is a cat it’s like you’ve never bought a full exhaust kit thinking it’s missing or knowing no name headers can make hp. So that comment you said 42 doesn’t even make sense to actually being something.

 
Rep Power: 222

Did your parents have any kids that lived?
Or could at least type things in English properly?

Whatever exhaust you bought either has no cat (a "test pipe") or a very low quality aftermarket cat.
The computer doesn't like this and that's why you're getting the p0420 code.
Riceboy told you your options.

 
Rep Power: 367

don't believe in all ads.
most people have a placebo effect, they think it got more power, but stopwatch tells otherwise.
In autoX, I thought i was at the maximum possible. Looking at the tire wear, decided to soften the front suspension half way.
Got more than a second faster in a 70 second course.
Next run, went all the way soft on the front. Got another second.

Car felt faster in the initial setup.
softer front felt uneasy, weird handling but ultimately faster - more than 2 seconds in a 70 second course is considerable

 
Rep Power: 367

anyway, the CAT is likely not up to the job and will throw a code.
Or there might not be a CAT or very low quality

 
Rep Power: 511

A lot of times they put test pipes that "look" like catalytic converters in hopes that they'll pass visual inspections for states that require emissions testing, and it's up to the end user to make it so it'll pass the smog check.

O2 simulator if you need the CEL to be off, or let it be and periodically check for actual codes if your county doesn't require emissions testing. I had a high flow cat on my 2002 that actually did work, but since it was a high flow cat, threw a p0420 code after some time.

 
Rep Power: 0

[QUOTE=Colin42;4787970]Did your parents have any kids that lived?
Or could at least type things in English properly?

Whatever exhaust you bought either has no cat (a "test pipe") or a very low quality aftermarket cat.
The computer doesn't like this and that's why you're getting the p0420 code.



yes that’s why that writing is better. To say it’s not it’s just trying to really say it’s not cuz well idk what reason you have. But maybe check your insult cuz the writing will still be good and the kid it’s not your kit for sure it wanting to make something where there is nothing to make. Pointless you can say. I would say your pointless rather.

 
Rep Power: 511

keep it civil, guys, or I'm locking the thread.

 
Rep Power: 367

for a proper catalyzer, you will need to look for a 49 or 50 state legal sticker.
yes, there are full exhaust kits that don't have cats, FYI...

ah, the dog reference is just a joke about cats and dogs... No need to get upset...