CEL P0421, dealer blames exhaust - Mazda 6 Forums : Mazda 6 Forum / Mazda Atenza Forum
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post #1 of 42 (permalink) Old 10-10-2018, 10:38 AM Thread Starter
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CEL P0421, dealer blames exhaust

2016 6 touring with only 22k miles and a few days ago got a CEL P0421 "warm up catalyst efficiency below threshold". No apparent drivability issues, maybe slightly less power but might just be in my head due to the CEL being on.


Took it to dealership and they said it just needed a software update to the PCM (did some searching and found this https://mazda.oemdtc.com/376/check-e...014-2016-mazda). Picked it up and as I was driving back to work the check engine light came back on with the same code.


Now the dealership is claiming that it is caused by my aftermarket muffler having too little back-pressure?!


Has anybody heard of this, seems like BS! I've had the aftermarket muffler on there for 6 months without issue, and been loving the better sound.


Thanks
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post #2 of 42 (permalink) Old 10-10-2018, 11:03 AM
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They picked the easy thing to blame. But if that the issue, a lot of vehicles with aftermarket exhaust would have this issue.

My suggestion is to google and see what you find. I've never heard of this conclusion before personally.

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post #3 of 42 (permalink) Old 10-10-2018, 11:17 AM
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I'm curious why they don't replace the cat that is causing this code to trigger.

If they are trying to avoid doing so and blaming the exhaust (voiding the warranty) it is up to them to prove that is the cause.
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post #4 of 42 (permalink) Old 10-10-2018, 11:21 AM
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The problem now is that they can actually deny you any warranty claims on the exhaust system because it has been inspected at a dealership and found to have aftermarket parts installed. Whether you like it or not, aftermarket parts can and do void warranties.


Now I don't believe that the exhaust system is the issue, but you will not likely get anywhere with the dealer on this now. Try reaching out to Mazda themselves, don't be a jackass. Mazda cannot control dealers and dealers are not the people warrantying your car. Mazda is the one that covers the warranty, not dealers. Dealers are just licensed to buy/sell/repair new/used Mazda vehicles.


Make sure you don't develop an attitude with the dealer, they will probably stop working with you at all if you do. The dealer does not need to determine cause once they find aftermarket parts in the system they are inspecting.
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post #5 of 42 (permalink) Old 10-10-2018, 12:08 PM Thread Starter
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Well I took it back to the dealer this morning and asked them to try again. They said they would need to do more diagnostic and if it is found to be caused by aftermarket part then I will have to pay for it, but if it is under warranty then I won't have to pay for the extra diagnostic work. This implies to me they were trying to avoid work by just updating the PCM and assuming it was fixed, so they didn't have to test the catalytic converter or oxygen sensors.


There's another dealership 30 minutes away that a friend with a Mazda claims is far better for service, may try taking it there if I can't get anywhere with the closest one.



Will update when I hear back.
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post #6 of 42 (permalink) Old 10-10-2018, 12:43 PM
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Quote:
Originally Posted by b1lk1 View Post
The problem now is that they can actually deny you any warranty claims on the exhaust system because it has been inspected at a dealership and found to have aftermarket parts installed. Whether you like it or not, aftermarket parts can and do void warranties.


Now I don't believe that the exhaust system is the issue, but you will not likely get anywhere with the dealer on this now. Try reaching out to Mazda themselves, don't be a jackass. Mazda cannot control dealers and dealers are not the people warrantying your car. Mazda is the one that covers the warranty, not dealers. Dealers are just licensed to buy/sell/repair new/used Mazda vehicles.


Make sure you don't develop an attitude with the dealer, they will probably stop working with you at all if you do. The dealer does not need to determine cause once they find aftermarket parts in the system they are inspecting.
Yes, everything above is true. TBH, and I know it's not what you want to hear, but having too little backpressure could definitely cause the cat to heat up incorrectly. I haven't ever heard of this happening but it makes sense in my head. Is it enough to void your warranty? Well at the end of the day that is up to Mazda. I'd say that if it were to go to court, they would have a good case. And they know most people won't take it that far. But if you can talk to the Mazda corporate office and convince them, the dealership will fix your vehicle under warranty (assuming your warranty hasn't already expired). The dealership just doesn't want to tell you they can fix it under warranty, then when they explain the situation to Mazda, have Mazda deny payment for their work because of the exhaust mod.

There was a post above that said it's up to them to prove that the aftermarket parts caused the issue...that's only true to a small extent. That statement gets thrown around a lot but I wouldn't rely on it. A dealership isn't going to do an A/B test putting different mufflers on just to prove they are right. They will say your modded car has a CEL, and these 10,000 unmodded cars don't. In the case of most car problems, there isn't much you can REASONABLY do to prove it one way or another. In your case, their claim makes sense. If they told you the problem was caused by your aftermarket subwoofer, that would be a weak claim.

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Originally Posted by WoodinvilleDan View Post
Well I took it back to the dealer this morning and asked them to try again. They said they would need to do more diagnostic and if it is found to be caused by aftermarket part then I will have to pay for it, but if it is under warranty then I won't have to pay for the extra diagnostic work. This implies to me they were trying to avoid work by just updating the PCM and assuming it was fixed, so they didn't have to test the catalytic converter or oxygen sensors.


There's another dealership 30 minutes away that a friend with a Mazda claims is far better for service, may try taking it there if I can't get anywhere with the closest one.



Will update when I hear back.
This is sadly pretty common, I've seen so many crooked mechanics charge for a full diagnostic and then all they do is fix a part at random. Hopefully they will do right by you.
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post #7 of 42 (permalink) Old 10-10-2018, 01:45 PM
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This is disconcerting to hear.


What kind of exhaust setup are you running?
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post #8 of 42 (permalink) Old 10-10-2018, 02:07 PM
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HMM....The Magnusson-Moss act is your friend in these matters.

Having said that, if for example, you had straight pipes behind the exhaust that would be a no-brainer as far as legitimizing their claim... Hoping you find an acceptable solution.
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post #9 of 42 (permalink) Old 10-10-2018, 03:33 PM
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I am going to chime in on this as well. Back pressure is a distinct possibility if it is too high but unlikely if too low. I have had valve damage (especially 4cyl) in a vehicle where there was a significant lowering of back pressure. Changes to this degree usually reduce road drive-ability as the power band is moved significantly higher in the rev range (think high RPM track car).


Generally speaking the catalytic converter is the point of highest resistance in the exhaust system making this the most significant modifier for combustion and engine operation


Catalytic converters die (are killed rather) when there is too much reactant (un-burned fuel). This extra reactant enters the catalyst chamber and combines with the catalyst. The reaction is exothermic and raises the temperature higher than can be achieved within the combustion chamber. This extra temperature further separates the exhaust into its base elements, (gasses and water). If you are adding too much fuel into the exhaust (misfire, leaking injector, incorrect timing, WOT, etc...) the catalytic reaction happens too fast and it overheats and destroys the matrix that the catalyst is attached to (the honeycomb lattice material inside called a ceramic brick). Sulfur smell is an overheating catalytic converter. When brief it is okay (WOT). When sustained (blockage or misfire) it is deadly.



I can attest personally to an increase in back pressure adversely affecting AFR but a reduction should be well within the operating range of the vehicle and its software package.


What happens if you live at high altitude where there is lower atmospheric pressure and therefor lower pressure at exhaust exit? Do all you Coloradans have to replace your catalytic converters regularly? I would guess not.


I am highly skeptical that a reduction in back-pressure would change the vehicles ability to meter incoming air and match the appropriate amount of fuel with it. An increase in pressure yes, but not a decrease at the muffler (with a muffler still installed).


I would bet the PCM flash was to correct an air/fuel or timing issue. Another shop will now not see an issue. It is also possible the converter was not correctly broken in initially.


Everything the @b1lk1 said about dealer interactions is good advice. The dealer/manufacturer is a business and they have to maintain viability. It is impossible to test all the various modifications that can be made to a vehicle so a line needs to be drawn somewhere. Try to be reasonable and hope for the best.


Maybe you could work a deal where you pay for the part and they cover the install. Make sure it is correctly broken-in.





Summary: I don't think your aftermarket muffler destroyed your catalytic converter but something did. If there is a combustion issue, a new catalyst will die as well.


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post #10 of 42 (permalink) Old 10-10-2018, 05:17 PM Thread Starter
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The exhaust I put on was simple just a Thrush turbo muffler (


) welded to the flange cut off the stock muffler (and obviously some piping & bends to fit) so it's bolted on and could be removed/changed if I wanted to go louder or quieter.


Dealership claims that the Catalytic converter and the "air fuel ratio sensor" (I think this is just wideband O2 sensor) have been damaged due to lack of back-pressure... To fix it they claim the cat & sensor need to be replaced and a stock muffler put back on, to the tune of $2,200! WTF


I told them I am definitely not authorizing the work, either way they're charging me $160 just for diagnosis. I am going to go elsewhere, this does not make sense.
Super frustrated!
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