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What Drives You?
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Discussion Starter · #1 ·
What the code means, technically:
Code Description: Warm Up Catalyst Efficiency Below Threshold (Bank 1)
From OBD-Codes.com

What does that mean?
Basically this means that the oxygen sensor downstream of the catalytic converter on bank 1 is detecting that the converter is not working as efficiently as it should be (according to specs). It is part of the vehicle emissions system.​

Symptoms
You will likely not notice any drivability problems, although there may be symptoms. The code may be more likely to occur after the engine has had numerous cold starts in the past 1-2 days.​

Causes
A code P0421 may mean that one or more of the following has happened:
  • The catalytic converter is no longer functioning properly
  • An oxygen sensor is not reading (functioning) properly
  • A sparkplug is fouled up
Possible Solutions
The simplest thing to do is to reset the code and see if it comes back.

Next step is to measure the voltage at the oxygen sensor on Bank 1 (the rear sensor, or the sensor after the converter). The voltage should be low and steady (~200mV, depends on the vehicle). If the voltage varies (e.g. 100mV to 600mV) then it's time to replace the catalytic converter. In fact, it would be a good idea to test each oxygen O2 sensor while you're at it.

One thing to note is that many vehicle manufacturers offer a longer warranty on emissions-related parts. So if you have a newer car but it's out of it's bumper-to-bumper warranty, there still may be warranty on this type of problem. Many manufacturers give a five year, unlimited mileage warranty on these items. It's worth checking into.​
What it probably is for your 4G, if you've had the car for +/- one year (2006 year model):

A recall was announced for specific 4Gs (up to: manufacture date unknown) that throw this code due to cold temp starts and operation. The ECU for vehicles that rolled out prior to the recall is extremely sensitive to the variance in voltage returned by the O2 sensors and the sensor heater wire.​

The fix:
ECU reflash, covered under warranty.​

Possible extensive damage:
As stated above, ignoring this code might cause perma-damage to the 1,1 cat. If the ECU reflash was performed, and the SES light comes on again soonafter (usually within the next week - about 1 to 5 cold starts), then a replacement cat is to be ordered, also covered under warranty.​
 

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What Drives You?
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9,409 Posts
Discussion Starter · #2 ·
By the way, I did forget to mention that though the OBD-Codes writeup says that there's no effect on performance, MIVEC doesn't engage as quickly, making the car feel a little more sluggish on the pickup, and suck up a little more gas. Plus you don't get to grin as much when the woad-headed Tibby driver thinks his itty-bitty Hunny is a close pit against your 4G - you know you like the feeling of getting pinned to your seat at 2500RPM thanks to that wonderful pair of secondary lobes.
 

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Diamond Dog
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I'm getting this code now. Does anyone have a link to this recall information? I only see 3 recalls for the 2007 gt and none of them are this one.

Thanks,
 

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iv had this code since iv had the car and mitsu kept sending me away and away saying it was my fault due to peformance parts lmfao the codes are p0431 two times and p0421 two times on the obd2 reader. at first it was every 600km and then 300km and now after erasing them im lucky to go 60km with out it coming back on and mitsu kept sending me away with out replacing them and im way over the km warranty now. they want $1000 for each converter. what should i do ?
 

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I'd be all like 4g! Waat?
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12,545 Posts
iv had this code since iv had the car and mitsu kept sending me away and away saying it was my fault due to peformance parts lmfao the codes are p0431 two times and p0421 two times on the obd2 reader. at first it was every 600km and then 300km and now after erasing them im lucky to go 60km with out it coming back on and mitsu kept sending me away with out replacing them and im way over the km warranty now. they want $1000 for each converter. what should i do ?
It's a techincal service bulletin TSB if the cats fail they HAVE to replace them up to 80,000 miles BY LAW in ANY state
 

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thanks peps for the feed back. i want to get them out of there and have straight through pipes, whats the cheepest way ? and im at 138,000 km i dont know what that is in miles lol they said upto 130,000km they would cover it but they kept giving me the run around and i passed that mark.
 

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Premium Member
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961 Posts
I've been getting this code too. The code comes on and lasts like 2-3 days then automaticly shuts off. Haven't had time to go and take it to the scumlord dealership, in which will most likely tell me it's my mods prematurely adding more exhaust causing the clog. But it this goes away on it's own, does that mean perhaps it's more towards the sensor and btw which side is bank 1, front or rear? And not to derail the thread, also have my ac light blink and not throw cold air? I do have a leak in my condensor, but even b4 the charge it never blinked blue and green...
 

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The problem is theres 3 possibilities of what could be wrong, the cats, the o2 sensor or maybe a spark plug. What they said is they got to run a diagnostic but if it comes back the cats then its covered, but if its not the cats then im stuck with the 110 for the diagnostic. I took my car back because I wasn't a 100% sure it was the cats, I guess I'm going to check the other options to make sure its the cats before i have them run it. But i was thinking about doing RRE headers and an exhaust anyways so idk yet.
 

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North 4G Korea dissident
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6,741 Posts
hey guys,
i installed an Ingall Stiffy yesterday... while i did f*** up 2 bolts, the thing that worries me is that right after the install i went for a test drive, let the car warm up, and then did a few hard take-offs to see the difference... all good... an hour later, the GF and I go for a ride, and i start noticing a tac-tac-tac kind of noise from the engine... seems to be on and off randomly... i stopped, checked the Stiffy and everything that i could have messed up around it, all looks good... shortly after, my SES light comes on, checked it this morning, P0421 (Bank 1).
the noise seems to be coming from the left-rear of the engine while looking at the engine bay... i'm lost as to what the link is between the Stiffy and P0421, if any? or just random bad luck?
any idea? it's an 08 GT, engine #2...

Pat
 

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ok.. i just got this code.. got a free check engine light scan from auto zone.. and they said the code pops up twice. anyone figure out what their problem was?? i have the aeon header back exhaust which takes out the main cat.

Edit: also should say that ive had the exhaust on for over a year and a half and this is the first time i got the SES light.. funny too cause i had just gotten an inspection done about a week before the light popped up.
 

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Theta II
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8,583 Posts
hey guys,
i installed an Ingall Stiffy yesterday... while i did f*** up 2 bolts, the thing that worries me is that right after the install i went for a test drive, let the car warm up, and then did a few hard take-offs to see the difference... all good... an hour later, the GF and I go for a ride, and i start noticing a tac-tac-tac kind of noise from the engine... seems to be on and off randomly... i stopped, checked the Stiffy and everything that i could have messed up around it, all looks good... shortly after, my SES light comes on, checked it this morning, P0421 (Bank 1).
the noise seems to be coming from the left-rear of the engine while looking at the engine bay... i'm lost as to what the link is between the Stiffy and P0421, if any? or just random bad luck?
any idea? it's an 08 GT, engine #2...

Pat
I don't see how installing a stiffy could foul a plug, break an 02, or a Cat since I am assuming you don't even have those two precats anymore?
 

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North 4G Korea dissident
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6,741 Posts
I don't see how installing a stiffy could foul a plug, break an 02, or a Cat since I am assuming you don't even have those two precats anymore?
I still have my stock headers, i guess they're just going bad, or maybe just the sensor... I dont think the stiffy had anything to do with it though. Just a bad coincidence!
 

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ok.. so i brought it in to my mechanic.. and they dont really know what the deal is other than its throwing that code.. so they THINK its something with the cat.. they called the dealer and gave them my vin and said that if its the cat a cat replacement SHOULD be covered.. then a pcu reflash or something.. but that wont be covered.. and then whatever for labor..

i for some reason just cant believe its my cat right now.. its an 06.. i dont over-rev frequently.. i do have the aeon header back exhaust that takes the main cat out tho.. but i dont wanna have to take the time to put my car back to stock before bringing it back there unless i know its def what i need to do.. has anyone with a fujita CAI put their stock box back in?? is it a bitch?? :/
 

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Theta II
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8,583 Posts
ok.. so i brought it in to my mechanic.. and they dont really know what the deal is other than its throwing that code.. so they THINK its something with the cat.. they called the dealer and gave them my vin and said that if its the cat a cat replacement SHOULD be covered.. then a pcu reflash or something.. but that wont be covered.. and then whatever for labor..

i for some reason just cant believe its my cat right now.. its an 06.. i dont over-rev frequently.. i do have the aeon header back exhaust that takes the main cat out tho.. but i dont wanna have to take the time to put my car back to stock before bringing it back there unless i know its def what i need to do.. has anyone with a fujita CAI put their stock box back in?? is it a bitch?? :/
There was a recall for a reflash for the 06 so Idk why they said it would cost you? However if you don't have any cats than your cat can't be broken. Spacer's will most likely fix it if that's the case
 
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