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Discussion Starter #1
I have a completely aftermarket exhaust system (see below) and I have been throwing the CEL code P0420 (Catalyst System Efficiency Below Threshold {Bank 1}). I bought an OBDII reader to clear this code (usually took a week or two to reappear) and after a while I got lazy and just left it on. I decided to fix the problem by adding an O2 defouler to the rear O2 sensor, which I heard should fix the issue. Today I finished that project and plugged in my OBDII reader to find out that the latest code that had been thrown (probably 2 or 3 months ago) was P0141 (O2 Sensor Heater Circuit Malfunction {Bank 1, Sensor 2}), which means that the O2 sensor has failed. I clear the code and it will reappear immediately after turning the engine back on. I am pretty sure that the O2 sensor is shot...but what I am curious about is whether the P0420 code was a warning that the O2 sensor was on its way out or do I actually need a defouler AND a new O2 sensor to keep from throwing another CEL. Is there anything else I should consider checking?

Also, I was amazed at the price of this O2 sensor ($250)! I found one made by Beck/Arnley for $65, I hope it doesn't give me any issues!

Any help or opinions are welcome...thanks again crew!
 

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iVTEC no koe
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That (secondary) O2 sensor is only $200+ at the dealer. You can get NTK/Denso's for low $100-120, or Bosch for $80-100. Personally, I would only go with the aforementioned brands for O2.

I also have a header and RT cat, but I've been running a defouler since the day I've put on those parts (after a couple months of buying the car) so my O2 hasn't really been exposed to any excessive exhaust fumes. Depending on how long you had it running with the aftermarket parts, you might have cut the life short on the O2.

After putting on the defouler, it may take a couple days (depending on how long your drives are) for the code to disappear by itself. Worst case, just replace the sensor.
 

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That's Doctor
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If you're sure you didn't mess something up during the install, this is probably just coincidence. I would keep the defouler and get a new sensor. Also, I've heard some bad things about aftermarket O2 sensors failing prematurely and to only buy Bosch or Denso. It's likely that the Beck is not a plug-n-play sensor too. Usually the $60 ones require that you cut and splice your old plug onto the new sensor. If it's the sensor on the cat, you'll need to remove all the grommets and clips and transfer them over to the generic sensor. The expensive sensors come assembled with all new grommets and plugs and clips etc.
 

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Evan
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I have a completely aftermarket exhaust system (see below) and I have been throwing the CEL code P0420 (Catalyst System Efficiency Below Threshold {Bank 1}).
Hey man, I actually get this same error alot too. Its hit and miss as well to when it comes back on after I reset the codes or pull the battery terminal. (can be from a few days to a couple weeks). Like you I have a completly aftermarket exhaust (headers and catback) however; the one item that is still stock is the CAT. So I have been thinking that the issue was with the sensor attached to the header and not the CAT. Either way im not sure and still rocking the codes...lol. I would love to hear updates with you get some.
 

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Thanks guys, I have changed my order to a Denso direct replacement O2 sensor...fingers crossed that this solves the issue. Would you suggest that I keep the defouler in to prolong the life of the new sensor...given my mods? Thanks again!
 

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Discussion Starter #6
Just got my new Desco O2 sensor in and no CEL codes! I have left the defouler in to reduce the heat exposure so we will see if anything shows up down the road. Thanks for the tips guys!
 

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That's Doctor
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That's great to hear. I've heard that even with a defouler, some do experience a few CELs here and there. Just something to watch out for.
 
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