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2012 Mazda 3 2.0L Automatic Sedan
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Discussion Starter #1
Hey there, I'm pretty new to any sort of modifications but I've been dealing with a CEL with a misfire code for a bit, replaced my spark plugs and coils and that didn't fix it. Took the car in to a shop and they said I needed a valve cover gasket, got that replaced along with the spark plugs again and still had a misfire in cylinder 3. Took it back again and they said I need a new cat converter.

Now my question is, if I pick up an aftermarket converter (I was looking at the magnaflow OBDII direct fit cat), can I also throw on a new exhaust altogether?
I guess cat-back just means it's the portion that attaches to the back of the cat as the name sounds?

I'll throw in links for the parts I'm currently looking at, any advice would be appreciated, sorry I'm really new to this 馃槄

Catalytic Converter for 2012-2013 Mazda 3 2.0L L4 GAS DOHC | eBay
2010 2011 2012 2013 Mazda 3 2.0L Magnaflow Catback Exhaust Free Shipping | eBay
 

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2012 Mazda 3 GX MT5 non sky
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595 Posts
In have this on my 2012 2.0L Mazda Exhaust System | 60mm Exhaust

Which Cat has to be replaced the main cat on the headers or the mid point cat (which is right after the downpipe and before the cat back exhaust.
 

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2012 Mazda 3 2.0L Automatic Sedan
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Discussion Starter #3
In have this on my 2012 2.0L Mazda Exhaust System | 60mm Exhaust

Which Cat has to be replaced the main cat on the headers or the mid point cat (which is right after the downpipe and before the cat back exhaust.
I actually hadn't realized that there was more than one, so I guess my next step before doing anything else should be to find out.
 

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2012 Mazda 3 2.0L Automatic Sedan
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Discussion Starter #4
I actually hadn't realized that there was more than one, so I guess my next step before doing anything else should be to find out.
Update: Called up the shop to ask and they didn't seem to think there was more than one cat so at this point I'm really not sure.
 

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2012 Mazda 3 GX MT5 non sky
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CEL will tell you where it is
 

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2013 Mazda 3 i Hatchback
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66 Posts
The only way a valve cover causes a misfire is if the seal around the plug tube goes bad and drowns the plug in oil. Possible, and changing the plug again is a good precaution. But I wonder if the coil is still oil-soaked or compromised. Might want to swap a couple of the coils and see if the misfire moves.

I'm having a hard time believing that a failing/plugged cat is causing a misfire only on cylinder 3. Seems to me it should be a random misfire showing up on multiple cylinders. Replacing a cat is an expensive repair (profitable for the shop), and you want to be sure it isn't something else before doing that.
 

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2012 Mazda 3 2.0L Automatic Sedan
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Discussion Starter #8
The only way a valve cover causes a misfire is if the seal around the plug tube goes bad and drowns the plug in oil. Possible, and changing the plug again is a good precaution. But I wonder if the coil is still oil-soaked or compromised. Might want to swap a couple of the coils and see if the misfire moves.

I'm having a hard time believing that a failing/plugged cat is causing a misfire only on cylinder 3. Seems to me it should be a random misfire showing up on multiple cylinders. Replacing a cat is an expensive repair (profitable for the shop), and you want to be sure it isn't something else before doing that.
I鈥檒l go ahead and give that a shot before I proceed with anything else. I can get the rear cat done relatively inexpensive but I鈥檓 not sure how much the manifold cat would run me. Already seeing that the part itself is quite a bit more expensive than a rear cat. Appreciate the advice!
 

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2012 Mazda 3 2.0L Automatic Sedan
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Discussion Starter #9
The only way a valve cover causes a misfire is if the seal around the plug tube goes bad and drowns the plug in oil. Possible, and changing the plug again is a good precaution. But I wonder if the coil is still oil-soaked or compromised. Might want to swap a couple of the coils and see if the misfire moves.

I'm having a hard time believing that a failing/plugged cat is causing a misfire only on cylinder 3. Seems to me it should be a random misfire showing up on multiple cylinders. Replacing a cat is an expensive repair (profitable for the shop), and you want to be sure it isn't something else before doing that.
So I went ahead and swapped the coils for 1 and 3, 3 is still showing a misfire and surprisingly I got a single misfire in cylinder 4 as well which hasn鈥檛 happened before. I鈥檓 not gonna lie I鈥檓 at a loss. I picked up some cat cleaner just on the off chance that that helps somehow but beyond that I鈥檓 not sure what else could be causing it. I鈥檇 like to cover all my bases before going nuclear and going ahead with a cat swap.

So far I鈥檝e replaced the spark plugs, coils, valve cover gasket, spark plugs again after the VCG.
 

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1- Stop throwing parts at the car. It expensive and gets you nowhere.
2- Find a shop that isn't as interested in taking your money and more capable of find the problem. If they don't even know how many converters are on your car, you got to wonder what else they don't know.....I can see how a continued misfire can cause a converter to fail, but not how a failing converter can cause a misfire on just one cylinder...
Have you checked the crank position sensor or anything else related to the timing and ignition?
 

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2012 Mazda 3 2.0L Automatic Sedan
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Discussion Starter #11
1- Stop throwing parts at the car. It expensive and gets you nowhere.
2- Find a shop that isn't as interested in taking your money and more capable of find the problem. If they don't even know how many converters are on your car, you got to wonder what else they don't know.....I can see how a continued misfire can cause a converter to fail, but not how a failing converter can cause a misfire on just one cylinder...
Have you checked the crank position sensor or anything else related to the timing and ignition?
I only went to that shop the one time and they did the VCG and the spark plugs, then suggested cat when I told them it didn鈥檛 fix the CEL, everything else I鈥檝e done myself; but in any case no I haven鈥檛 checked anything else yet. I鈥檝e got a little bit of know how but I鈥檓 still a novice so I鈥檓 not really sure what else could potentially cause it. I鈥檒l have to look into those next. Is it possible for the fuel injectors to be the cause?

I should also note that there鈥檚 a big difference in idle when I turn off the injectors during live tests, it definitely gets rough until they鈥檙e turned back on.
 

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2013 Mazda 3 i Hatchback
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On a 4-cylinder, a misfire on one cylinder is usually noticeable. If the engine doesn't idle rough, and the idle gets rough on all 4 as you unplug the injectors, then we can assume that the engine isn't misfiring at idle (at least not when you've been watching).

It sounds like you have an OBD reader, does this reader allow you to look at live data? If so, I'd be interested in looking at the short & long term fuel trim, and watching the misfire counter for what conditions produce misfires. Long term fuel trim should be close to zero after the car is warm. Short term will vary more, but once the car is warm and the load and throttle are stable it should also be close to zero. If they're not, it's a sign that the computer is trying to compensate for something.

We're focusing on the cats because a garage told you that was the problem. Keep in mind that there's a lot of potential causes of misfires. But let's explore the cat theory a bit.

There's three ways that a cat "fails": it either clogs up with soot, physically breaks apart internally, or it's got enough miles on it that it exhausts the supply of catalyzing elements. Mild clogging of the cat, or used up catalyzing elements will usually throw a code for "efficiency below threshold" or something like that. But extreme clogging, or the physical breakdown of the internal structure can create enough of an exhaust restriction that it causes misfires.

Some basic background questions: How many miles does the car have on it? Has it always been well maintained? If a previous owner drove it for 6 months with a bad plug, the cat could be ruined prematurely.

Have you noticed any points in driving it when it runs rough or lacks power? Trying to identify when those misfires are happening. If the car just generally lacks power, it might be a sign of clogged cats.

On cold startup, do you hear rattling from under the car? When the cat breaks down, the chunks of cat will rattle around like rocks in a can. In fact, with the car off, you can rap on the cats or the exhaust pipe near the cat with a mallet and see if you hear rattling.

Do you ever get the "rotten egg" smell? That's also a sign of a failing cat.

So, let's NOT take the garage's word on the cats and look at the other sources of a misfire: fuel, air, and spark.

Fuel. Since the car runs, we know you're getting fuel, but is it able to supply enough of it under all conditions? Is there a sticking injector that's either not delivering enough or too much fuel under some conditions? Is the fuel pump not keeping up at WOT? Is the fuel filter clogged? Have you gotten some bad gas (though I'd expect to have misfires on all cylinders from bad gas)?

Air. Again, it runs, so it's working at some level. If you have access to a compression tester, it might be interesting to see how #3 compares to the rest. Also, have you checked the air filter and looked at how dirty the throttle body is? Intake leaks can cause misfires, have you checked out all the intake/PCV connections?

Spark. You've changed the plugs and swapped around the coils, and it seems to idle fine, but as others have pointed out, the TIMING of that spark is important. A crank sensor or cam sensor usually just goes bad and the car won't start, but I suppose it's possible that it goes just a little bad and starts lying to the computer just enough to make it misfire under some conditions.
 

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2012 Mazda 3 2.0L Automatic Sedan
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Discussion Starter #13
On a 4-cylinder, a misfire on one cylinder is usually noticeable. If the engine doesn't idle rough, and the idle gets rough on all 4 as you unplug the injectors, then we can assume that the engine isn't misfiring at idle (at least not when you've been watching).

It sounds like you have an OBD reader, does this reader allow you to look at live data? If so, I'd be interested in looking at the short & long term fuel trim, and watching the misfire counter for what conditions produce misfires. Long term fuel trim should be close to zero after the car is warm. Short term will vary more, but once the car is warm and the load and throttle are stable it should also be close to zero. If they're not, it's a sign that the computer is trying to compensate for something.

We're focusing on the cats because a garage told you that was the problem. Keep in mind that there's a lot of potential causes of misfires. But let's explore the cat theory a bit.

There's three ways that a cat "fails": it either clogs up with soot, physically breaks apart internally, or it's got enough miles on it that it exhausts the supply of catalyzing elements. Mild clogging of the cat, or used up catalyzing elements will usually throw a code for "efficiency below threshold" or something like that. But extreme clogging, or the physical breakdown of the internal structure can create enough of an exhaust restriction that it causes misfires.

Some basic background questions: How many miles does the car have on it? Has it always been well maintained? If a previous owner drove it for 6 months with a bad plug, the cat could be ruined prematurely.

Have you noticed any points in driving it when it runs rough or lacks power? Trying to identify when those misfires are happening. If the car just generally lacks power, it might be a sign of clogged cats.

On cold startup, do you hear rattling from under the car? When the cat breaks down, the chunks of cat will rattle around like rocks in a can. In fact, with the car off, you can rap on the cats or the exhaust pipe near the cat with a mallet and see if you hear rattling.

Do you ever get the "rotten egg" smell? That's also a sign of a failing cat.

So, let's NOT take the garage's word on the cats and look at the other sources of a misfire: fuel, air, and spark.

Fuel. Since the car runs, we know you're getting fuel, but is it able to supply enough of it under all conditions? Is there a sticking injector that's either not delivering enough or too much fuel under some conditions? Is the fuel pump not keeping up at WOT? Is the fuel filter clogged? Have you gotten some bad gas (though I'd expect to have misfires on all cylinders from bad gas)?

Air. Again, it runs, so it's working at some level. If you have access to a compression tester, it might be interesting to see how #3 compares to the rest. Also, have you checked the air filter and looked at how dirty the throttle body is? Intake leaks can cause misfires, have you checked out all the intake/PCV connections?

Spark. You've changed the plugs and swapped around the coils, and it seems to idle fine, but as others have pointed out, the TIMING of that spark is important. A crank sensor or cam sensor usually just goes bad and the car won't start, but I suppose it's possible that it goes just a little bad and starts lying to the computer just enough to make it misfire under some conditions.
Let me start out by saying I appreciate how thorough and well thought out your response is.

My family owns a used car dealership so I鈥檝e got access to a maxisys scanner which is how I鈥檝e been checking my codes.

I鈥檒l drop by the lot and take a look at the live data as you鈥檝e suggested when I have a minute to do so.

As far as previous owners go I unfortunately don鈥檛 know, as far as I know it was previously a fleet vehicle that ended up at auction, family dealership picked it up from there and I bought it from them. It鈥檚 fairly high mileage though at 173k.

As for notable lack of power, I can鈥檛 say I鈥檝e noticed one. Every now and then, it鈥檚 not often but I鈥檒l start driving and the engine will seemingly begin to stall. The traction control light will start flashing and it just seems like it鈥檚 losing power. Outside of this I would say it runs completely fine, and again this happens maybe 1/10 drives at most, not to say it鈥檚 not a serious issue but I figure I may as well let you know the frequency.

As far as air goes, I actually just ordered a cork sport SRI so once that comes in it鈥檒l be a good opportunity for me to inspect the throttle body.

I suppose with regards to the crank position sensor the best way to test that would be to check voltage/resistance on the sensor itself?

Again I really appreciate the assistance, I鈥檇 prefer not to take it back to a shop and sink money into replacing every potential suspect rather than doing the proper troubleshooting to figure it out.
 

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2012 Mazda 3 2.0L Automatic Sedan
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Discussion Starter #14 (Edited)
On a 4-cylinder, a misfire on one cylinder is usually noticeable. If the engine doesn't idle rough, and the idle gets rough on all 4 as you unplug the injectors, then we can assume that the engine isn't misfiring at idle (at least not when you've been watching).

It sounds like you have an OBD reader, does this reader allow you to look at live data? If so, I'd be interested in looking at the short & long term fuel trim, and watching the misfire counter for what conditions produce misfires. Long term fuel trim should be close to zero after the car is warm. Short term will vary more, but once the car is warm and the load and throttle are stable it should also be close to zero. If they're not, it's a sign that the computer is trying to compensate for something.

We're focusing on the cats because a garage told you that was the problem. Keep in mind that there's a lot of potential causes of misfires. But let's explore the cat theory a bit.

There's three ways that a cat "fails": it either clogs up with soot, physically breaks apart internally, or it's got enough miles on it that it exhausts the supply of catalyzing elements. Mild clogging of the cat, or used up catalyzing elements will usually throw a code for "efficiency below threshold" or something like that. But extreme clogging, or the physical breakdown of the internal structure can create enough of an exhaust restriction that it causes misfires.

Some basic background questions: How many miles does the car have on it? Has it always been well maintained? If a previous owner drove it for 6 months with a bad plug, the cat could be ruined prematurely.

Have you noticed any points in driving it when it runs rough or lacks power? Trying to identify when those misfires are happening. If the car just generally lacks power, it might be a sign of clogged cats.

On cold startup, do you hear rattling from under the car? When the cat breaks down, the chunks of cat will rattle around like rocks in a can. In fact, with the car off, you can rap on the cats or the exhaust pipe near the cat with a mallet and see if you hear rattling.

Do you ever get the "rotten egg" smell? That's also a sign of a failing cat.

So, let's NOT take the garage's word on the cats and look at the other sources of a misfire: fuel, air, and spark.

Fuel. Since the car runs, we know you're getting fuel, but is it able to supply enough of it under all conditions? Is there a sticking injector that's either not delivering enough or too much fuel under some conditions? Is the fuel pump not keeping up at WOT? Is the fuel filter clogged? Have you gotten some bad gas (though I'd expect to have misfires on all cylinders from bad gas)?

Air. Again, it runs, so it's working at some level. If you have access to a compression tester, it might be interesting to see how #3 compares to the rest. Also, have you checked the air filter and looked at how dirty the throttle body is? Intake leaks can cause misfires, have you checked out all the intake/PCV connections?

Spark. You've changed the plugs and swapped around the coils, and it seems to idle fine, but as others have pointed out, the TIMING of that spark is important. A crank sensor or cam sensor usually just goes bad and the car won't start, but I suppose it's possible that it goes just a little bad and starts lying to the computer just enough to make it misfire under some conditions.
So I didn鈥檛 have time to do anything yesterday but I鈥檓 running live tests today and at idle I鈥檓 seeing that my throttle position sensor is reading at about 14%. I鈥檓 no expert but that doesn鈥檛 sound normal for idle. Any thoughts?

I checked the fuel trim values as well, attaching pictures.

Edit: I kept the live test running and applied some throttle and while it felt smooth my readings show misfiring on all cylinders except for 4, mostly on cylinder 3. The other cylinders haven鈥檛 shown misfires at idle, only when throttle is applied, mostly upwards of 5k rpm
 

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So I didn鈥檛 have time to do anything yesterday but I鈥檓 running live tests today and at idle I鈥檓 seeing that my throttle position sensor is reading at about 14%. I鈥檓 no expert but that doesn鈥檛 sound normal for idle. Any thoughts?
That depends on what the ECU is reading. Absolute and relative positions are read differently. 14% should be about right for an absolute position reading but relative would be much less, close to 0%

Looking at the 4th picture added it does appear you definitely have some issue with #3 cylinder. You say you have put new coils on, and swapped coils around, but neither made any real difference. Have you checked the coil wiring and harness plugs for damage or corrosion?
 
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