typukatch

P0421 Code And Engine Hesitation...

8 posts in this topic

Have been doing some reading about the P0421 codes on here.

 

I got one a few months ago. It came and went a few times.

 

Seems like it could be my downstream O2 sensor, judging from the other posts on here. Or, it could be something else (vacuum leak? cat itself? precat? a timing belt? spark plugs?)

 

However, the car hesitates more and more now (since last month) and struggles to get out of 2nd gear but still has plenty of power after the hesitation. Would a faulty downstream O2 sensor still cause this?

 

It is a 2000 Mazda 626 ES V6 with 230k miles.

 

The EGR valve was cleaned two months ago and all the sparkplugs were replaced with NGK plugs about two years ago.

 

Your thoughts would be MUCH appreciated.

 

Thank you so much,

 

Ty

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You should take it a mechanic that has a live data scan tool or purchase one for yourself. (ELM327+Smartphone App works well enough).  First thing you should do for a P0421 is check your fuel trims.  If you want to know how to interpret fuel trim data please look up a video by ScannerDanner on YouTube called "Understanding Short and Long Term Fuel Trims.

 

 

Here's another good one

 

 

With a P0421 your downstream O2 sensor (farthest from the engine) is telling the ECU that the upstream O2 sensor (closest to the engine) that there is a big discrepancy in the catalytic converters ability to do it's job properly in scubbing emission gasses.  It could be as simple as an actual bad catalytic converter or it could be a false-positive like a blown exhaust gasket allowing oxygen into the exhaust pipe from a leak.  The issue here is to find out exactly what is causing that code to set by gathering evidence and confirming a diagnosis.  There are plenty of YouTube videos that can give you a general idea on how to attack a P0420 or P0421.  You could use both codes to figure out your issue.  The P0420 is a more common code so you'll likely find more info on it.

 

Again, this is all just a first step, a step that a technician would take to immediately start diagnosing your issue.  Depending on the results of the test will depend on the next step you take. Hope that helps get you started and headed in the right direction.  Every case is different and this is what you should be doing in YOUR case.

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Hey DJ, thanks so much for your quick response!

 

I have taken your advice and used Torque Pro with an ELM327 device, with the following outputs (while the car is in drive with my foot on the brake):

 


 

Also, I am terribly sorry, I originally said that it was an 0421 error, when it was actually an 0431 (Warm Up Catalyst Efficiency Below Threshold Bank 2) error.

 

As luck would have it, it appears that I also have a returning 0400 (EGR) code now too, lol

 

Anyways, I tried to add as many gauges as I thought would be pertinent to our issue here. Some don't get readings.

 

For the "in focus" items, per your response, the key:

 

Fuel Trim:

 

LTFT1 is long term fuel trim for bank one

 

F/T 1X1 is fuel trim bank one sensor 1

 

STFT1 is short term fuel trim for bank one

 

LTFT2 is long term fuel trim for bank two

 

F/T 2X1 is fuel trim bank two sensor 1

 

STFT2 is short term fuel trim for bank two

 

**NOTE: be sure to check out the second set of these readings in the video, at 2:43... for whatever reason, the long term fuel trim for bank two gets a reading the second time but not the first time.**

 

 

Oxygen Sensors:

 

O2 1x1 is Volts bank 1 sensor 1

 

O2 1x2 is Volts bank 1 sensor 2

 


O2 2x1 is Volts bank 2 sensor 1

 

O2 2x2 is Volts bank 2 sensor 2

 

 


 

I took a look at the videos you provided, and based on them, it appears that my fuel trim is okay and that my oxygen sensors would be working?

 

Although I don't know why I'm not getting a long term fuel trim 1 reading?

 

I maybe should have revved it more in park, like in the videos you provided above, to get a better idea of a potential vacuum, leak, but I honestly believe that this would not be the problem as the vacuum gauge shows a tight vacuum and I don't have a terrible loss of power. Likely could be wrong, though.

 

Here is another video I took of the car revving and really struggling to get out of second gear (first to second gear appears to shift fine):

 


 

I think this particular problem is indicative of a faulty neutral safety switch, because the car will lose its idle and stall if I stop suddenly and also takes a bit of time to switch gears from drive/park/reverse. It also appears, from this forum and others, that a faulty neutral safety switch can also cause the things just mentioned and uncontrolled revving.

 

Or perhaps something to do with the TPS sensor?

 

In any event, I'm sure the 0431 code (and the EGR code for that matter, too), all have something to do with the shifting problem as well.

 

What do you think?

 

Ty

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Take the torque app with a grain of salt.  Some of those values might not be correct.  I'm not sure if I believe them because according to those numbers you have the best running KL engine I've ever seen.  The ECU targets LTFT and STFT at 0.0 and your engine is the closest to 0.0 I've ever seen.  According to those numbers you do not have a vacuum leak or a problem of any kind.  Some snap throttles to verify the accuracy of the gauges would be a good idea yes.  Slowly accelerating to 3K and down again would help too.  It's just for a baseline to compare against idle parameters. 

 

See if you can find a PID that confirms you are in closed loop.  Closed loop = real time data.  Since your thermostat was at 88C (190F) it's just beginning to open but hasn't fully opened yet.  The thermostat will fully open at 96C (205F).  This means your engine has not fully warmed up yet when you were taking these readings.  If the engine hasn't fully warmed up then you are in open loop strategy (factory pre-planned idle strategy not using live data from sensors). 

 

Everything else looks pretty good though.  Your MAF g/s is a little low and your vacuum is really high.  You should be more around 16-18 inHg of vacuum.  That's why I'm a little suspicious about the accuracy of some of those numbers.  This is the first KL I've seen do 20 inHg of vacuum.

 

Being able to record the live output and share it to this site is awesome.  That will really help speed up the quality of help we can offer you.  Great job this is awesome.  Wish everyone could do something like this.

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Ask and ye shall receive!

 

I also put all of the real time data gauges together on one convenient screen so you can see everything at once.

 

I also realized that my app was set to a 2.0 liter displacement instead of 2.5 liters, so that might have explained some of the funny readings which you mentioned... my apologies on that:

 

https://drive.google.com/file/d/0B3OMmL31IzRHRUtscExSTUxVQUU/view?usp=sharing

 

Looks like it's in a closed loop... and that my vacuum goes down a bit and that the fuel trim gaps widen slightly on acceleration.

 

I start revving it in park around :40.

 

I also put the temperature in fahrenheit for your convenience.

 

I have been looking at a lot of your other posts on here, and I think I need to get in there again, given the car's miles (233k), and just make sure to clean all of the EGR ports a lot again. Cars this old get really dirty a lot easier. Last time I cleaned the EGR ports they were completely clogged.

 

Or maybe I have a slight vacuum leak somewhere.

 

As always, would love your thoughts.

 

Thank you again, DJ (and everyone else here),

 

Ty

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Looks normal to me.  Not sure how much I trust that data it's kind of all over the place.  You should expect both banks to peg rich.  The response time on the O2 sensors with that app just aren't good enough.  It's better than nothing but not definitive if looking for a fuel trim issue.  Nothing really jumped out at me as being obviously wrong.  The vacuum is a bit high that's about it.

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On 7/31/2015 at 2:42 PM, djdevon3 said:

Take the torque app with a grain of salt.  Some of those values might not be correct.  I'm not sure if I believe them because according to those numbers you have the best running KL engine I've ever seen.  The ECU targets LTFT and STFT at 0.0 and your engine is the closest to 0.0 I've ever seen.  According to those numbers you do not have a vacuum leak or a problem of any kind.  Some snap throttles to verify the accuracy of the gauges would be a good idea yes.  Slowly accelerating to 3K and down again would help too.  It's just for a baseline to compare against idle parameters. 

 

See if you can find a PID that confirms you are in closed loop.  Closed loop = real time data.  Since your thermostat was at 88C (190F) it's just beginning to open but hasn't fully opened yet.  The thermostat will fully open at 96C (205F).  This means your engine has not fully warmed up yet when you were taking these readings.  If the engine hasn't fully warmed up then you are in open loop strategy (factory pre-planned idle strategy not using live data from sensors). 

 

Everything else looks pretty good though.  Your MAF g/s is a little low and your vacuum is really high.  You should be more around 16-18 inHg of vacuum.  That's why I'm a little suspicious about the accuracy of some of those numbers.  This is the first KL I've seen do 20 inHg of vacuum.

 

Being able to record the live output and share it to this site is awesome.  That will really help speed up the quality of help we can offer you.  Great job this is awesome.  Wish everyone could do something like this.

There is also a transmission range sensor I believe that causes the tranny to over Rev.  

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If you get a catalytic converter below threshold code,  your cats toast. You need to replace it. Also important to find what killed it.

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