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2008 mazda 6 CEL

So I have a CEL on my 2008 mazda6i 2.3 liter. The Mazda mechanic said that the code was for a bad catalyic converter. But I used a Snap-on Ethos Scanner and with the snap on scanner it says “TFP Sensor/Switch “A” Circuit Range/Performance” and stated nothing about my catalyic converter.

Now I live in California so a smog check is a must. But I’m unsure which one to believe, sure I can replace one at a time and see which one it was, but the cost for labor and part for a catalyic converter in California is a lot of money.

What is the code number your reader listed? it would be in the form of “P1234”.

How many miles on this car? Any past history of problems? Oil changed on schedule?

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On a obd2 scanner it says “P0421 warm up catalyst system efficiency below threshold”. I have about 110,000 miles on this car, No history of past problems, Oil change is on schedule.

It’s the part where the high end snap-on Ethos scanner says something about my transmission A circuit. While a obd2 scanner says the P0421.

The OBD2 scanners don’t tell me about my transmission circuit, only the P code. While the Snap On ethos tool tells me about my Circuit not my catalytic converter.

Where did you get the Snap-on Ethos scanner? If borrowed, does the person you borrowed it from know how to use it?

A simple OBDII scanner will read just emissions codes. The more advanced scanner will read multiple computers in your vehicle system.

What I would guess happened is this:
You had the code read- P0421- and then the person that read it, cleared it too. You then went and found someone with the Snapon scanner, and they read the codes. Since the P0421 was cleared, it isn’t there anymore, and has yet to come back. But the Snapon reader found other issues going on.

Someone from california can verify this, but the transmission code should not keep you from passing a Smog test. If the other code was indeed cleared, and the system hasn’t gone thru a drive cycle, however, your car won’t pass.
However, if that transmission code is active, it should probably be checked out.

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Negative, I used the snap on tool at my community college mechanics school. Mazda dealership did not clear the code because they won’t clear anything that they don’t have the authorization to fix. After mazda I used my own odb2 scanner and read the same code that Mazda did. I did try to clear it with my scanner, but after a drive cycle the CEL came back on.

So then I used a Snap On Ethos tool at my school with basic training. So with the Snap On gave me a different code. So then I went back home after class and reused my same OBD2 scanner and still had the P0421 code but not the transmission circuit code.

I would think the “Snap On Ethos tool” should read the P0421 code. Perhaps someone more knowledgeable could verify this?

But if true, that means the tool is defective or being used incorrectly.

Probably the OBD2 scanner you used will not read transmission codes.

Well if my personal OBD2 scanner didn’t read the transmission code. Then how come the reader at the Mazda dealership didn’t read it either?

That’s telling you there’s a problem with the Transmission Fluid Pressure sensor

Any descent OBDII code reader should be able to read a P0421 and a P0841 DTC.


I get what it’s telling me. But I’m telling you one scanner is telling me something different from another scanner.

Both scanners are not picking up 2 Pcodes. 1 scanner says P0421, and Snap on Says P0841. No scanners are telling me both. Which is why I’m confused.

Did you enter the VIN of your vehicle in the Snap-On scanner?

The Snap-On Ethos scanner is VIN specific for Asian vehicles.


Yes I did enter my VIN. I set it to the car manufacturer, the model, the engine size and the transmission type. I didn’t select a bmw and proceeded to have it scan.


Now you got me confused.

It doesn’t make any sense!


Yes, the Ethos should read both codes.

I, too, am confuzzled…:confused:

If the software in the scan tool is older than the car you can get misinformation like that. Check the software level in the scan tool, the school may not be updating their equipment.

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This is something that I did not check. But then again if the software wasn’t up to date. Would it really give my 2008 mazda 6 as an option?

And no personality key was used. Due to the fact that snap on did not say insert key.

The transmission fluid pressure is supposed to be proportional to the vehicle speed, and it has a sensor apparently to see if it does. It decides what gear to shift to based on the vehicle speed, which it gets from the fluid pressure. Something wrong w/all that, that’s what that transmission code probably means. If the transmission is shifting ok, behaving like it should, then the problem can probably be deferred, either the sensor is out of calibration, or the pressure isn’t being regulated accurately enough to meet the diagnostic system’s requirements, which could mean you just need a proper transmission service.

The other problem, the p0421, that’s the one you should focus on resolving, b/c CA emissions definitely won’t let you renew your registration with that one. The way it works, there’s two o2 sensors installed and monitored by the engine computer, one before the cat, and one after. If the cat is working, that effects the o2 content from the output of the cat, so the sensor outputs will be different. Your mechanic is saying the two are “mirroring” each other, which is another way of saying the pre cat o2 and the post cat o2 output are the same. No good, you are probably looking at needing a new cat.

Why the scanners are not consistent, hard to say. It’s like asking why the desk top computer you used yesterday, doesn’t work today. Hardware compatibility problems, software compatibility problems, power supply problems, etc. all could cause that sort of thing. Suggest to rely on what the Mazda specific scan tool says.

Okay, the one thing about my transmission is that I don’t feel any issues with gear changing. There’s no stutter in gear changing or RPM’s going berserk