P0421 cel

RichB

Member
:
2007 Cosmic Blue Mazdaspeed 3 GT
I have been recently getting a P0421 code and am looking for idea of what it could possibly be? I have a stock downpipe with a CS Race pipe. I could only find posts about this code when people have an aftermarket downpipe installed.

Could this be because it started to get cold around here? I doubt it is my Cat or the O2 sensors going bad but let me know what you think.

Opinions are appreciated. Thanks.
 
:
2008.5 MG Mazdaspeed 3 GT
I have been recently getting a P0421 code and am looking for idea of what it could possibly be? I have a stock downpipe with a CS Race pipe. I could only find posts about this code when people have an aftermarket downpipe installed.

Could this be because it started to get cold around here? I doubt it is my Cat or the O2 sensors going bad but let me know what you think.

Opinions are appreciated. Thanks.

its a cat inefficiency code, it knows your missing a cat. the code is coming from the 2nd o2 sensor
 

blake_peanut

Member
:
2008 CWP MS3
I get the same code with my CP-E Catted TBE. Nature of the business I'm told. I bought an OBD II reader to clear the code. Apparently it will be triggered more so in the colder weather.
 

RichB

Member
:
2007 Cosmic Blue Mazdaspeed 3 GT
It knows I'm missing a cat even though the cat that is missing is after both sensors? Weird.
 
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All it "knows" is what it calls "cat temp" which is really secondary O2 temp. With high flow exhaust, this temp drops below a preset minimum and triggers the code. That's why it happens more in colder weather.
 

RichB

Member
:
2007 Cosmic Blue Mazdaspeed 3 GT
I appreciate the clarification. So a more efficient exhaust (heat away from engine faster) causes a CEL....Go figure!
 
Yeah, this is part of the balancing act any exhaust design has to work out.

What spins the turbine in a turbo car is flow rate of exhaust, yes but, primarily, it's the expansion of the heated gases in the exhaust pipe. A system that rejects too much of this heat too fast is giving away energy that the turbine can use.

OTOH, a system that backs up the flow too much can cause turbine stall and very poor performance.

Balancing these, along with emissions and noise regs, make car engineers work hard.

I haven't had a code yet in my catless DP/RP setup (Corksport) and I live in a colder clime (Canada). It seems every car is a little different, as is every environmental condition so, circumstances have to match up to set this code. It really doens't mean anything anyway, clear it and enjoy life.
 

RichB

Member
:
2007 Cosmic Blue Mazdaspeed 3 GT
I haven't had a code yet in my catless DP/RP setup (Corksport) and I live in a colder clime (Canada).

You don't get a code with your CS set-up because they extend the second O2 sensor housing so it isn't directly in the exhaust stream. This code will never get tripped with it like that.
 

matsuda

Member
All it "knows" is what it calls "cat temp" which is really secondary O2 temp. With high flow exhaust, this temp drops below a preset minimum and triggers the code. That's why it happens more in colder weather.

"Cat Temp" is not measured by any sensor (i.e. there is no "cat temp" sensor).

It is an estimate that is "calculated" by the ECU based primarily on engine load.
 

matsuda

Member
It knows I'm missing a cat even though the cat that is missing is after both sensors? Weird.

It doesn't know.

That code is unusual if you only have a test pipe and may indicate a bad secondary O2 sensor or a problem with the primary cat.
 
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2008.5 MG Mazdaspeed 3 GT
not sure bout that, my dh can give me cat temps to the 1/10th of a degree and it varies quite a lot. seems like an actual readout
 

CorkSport

2010 Mazdaspeed 3
Authorized Vendor
:
All Mazdas!
We have seen some variations in exhaust velocity cause some codes to trigger but that is normally with a downpipe installed

If you are running the stock downpipe and you are getting a code for the catalyst do you have a way to monitor the O2 voltages with a code reader or dash hawk? You can watch to see if the second O2 sensor is fluctuating the voltage to make sure it is working. If the voltage is steady or has very little variation the O2 sensor is shot. If the voltage is changing with a wide variation in voltage the cat is not heating up properly which can be a dead cat, tune issue, etc, etc.

-Derrick
 

matsuda

Member
not sure bout that, my dh can give me cat temps to the 1/10th of a degree and it varies quite a lot. seems like an actual readout

There is no cat temp sensor. The "cat temp" is "made up" by the ECU (which does a pretty good job of simulating the real cat temp).

This is a quote from the SAE J1979 (OBDII) specification:

"CATEMP11 shall display catalyst substrate temperature for a bank 1 catalyst, if utilised by the control module strategy for OBD
monitoring, or the Bank 1, Sensor 1 catalyst temperature sensor. CATEMP11 may be obtained directly from a sensor, or may be
inferred by the control strategy using other sensor inputs."


Try this: With the engine fully warmed up, monitor the "cat temp". Then quickly turn off the engine and start it again.

Notice anything strange?
 
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RichB

Member
:
2007 Cosmic Blue Mazdaspeed 3 GT
I have since taken out my test pipe and put the stock mid pipe back in and haven't seen the CEL yet. It also has been a little warmer so I'm interested in seeing what happens when OATs get in the 20s again.
 

MSMS3

Member
The second O2 sensor's "read" is pretty sensitive to changes in exhaust mods. I was unable to get this code to go away with the extended second 02 bung on my TurboXS dp/rp, nor could I get it to reliably stay away with a spark plug extender screwed into that bung between the pipe and the sensor. It helped, but still the code came back pretty frequently and would then go away at times.

What did work is the "diode fix:

http://www.mazdas247.com/forum/showthread.php?t=123716776

You have to cut and splice the black wire going to the sensor. Some may not want to do this. But for me, this proved to be the solution.
 
I was definitely happier with Corksport's solution than having to cut and splice a new car's wiring. I was manning up for that but, fortunately, it was unnecessary.
 

RichB

Member
:
2007 Cosmic Blue Mazdaspeed 3 GT
Well......my code came back even with the stock midpipe on. Even though i'm not getting a misfire code, I checked all of my spark plugs and they look good. Random question but can failing coil packs cause misfires without popping a misfire code?

Anyway, my plan now is to change the rear O2 sensor. I hope that fixes my problem.