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Discussion Starter #1 (Edited)
Good morning everyone. Haven't seen one of these around here yet, and there aren't too many of us at the point where we need to do this, so here's a quick reference guide.

This is for replacing a 2014, 2015, 2016 or 2017 Mazda6 Skyactive 2.5L Spark Plugs.

The total process only takes 10 - 20 minutes depending on if you like to do all 4 plugs at once, or 1 at a time; I prefer the latter.

Things You'll Need:
  • Replacement Spark Plugs - I recommend stay away from cheap Denso from Amazon.
    • Part Numbers:
    • Boche: 9621
    • Denso: 4711
    • NGK: 94124
  • 14mm Deep Socket (ideally, one with a magnet for removing plugs - seen in photo).
  • 8mm Socket or Wrench, but socket works better.
  • Socket Extension, yes even if you have the plug removal tool.
  • Anti-seize (not necessary, but recommended).
Spark Plug Gap per the Manual:

Spark plug gap [PE01-18-110, PE02-18-110]
Standard: 0.75—1.10 mm {0.030—0.043 in}
New spark plug (reference): 0.75—0.85 mm {0.030—0.033 in}

Spark plug gap [PE5R-18-110, PE5S-18-110]
Standard: 1.05—1.40 mm {0.0414—0.0551 in}
New spark plug (reference): 1.05—1.15 mm {0.0414—0.0452 in}



1. Open your hood (duh) and remove the engine cover by pulling up on it until the rubber plugs pop out.

2. Locate the 4 ignition coils and their individual 8mm nuts (Seen Below):


3. Unplug the wire connector from the coil(s) that you plan to work with. Simply press the plastic center piece down and pull the plug out.

4. Unbolt the 8mm nut from the coil.


5. Pull the coil straight up and out. Should come out without much force at all. (Sorry, no photo for this one).

6. You should now be able to see the plug waaaaayyy down. Time to get it out with the 14mm deep socket.


7. This is where a magnetic wrench or spark plug socket REALLY come in handy. It's hard to tell, but I counted at least 15 full rotations for the plug before it could be pulled out.


8. Compared to the new one it's not looking too bad for 83k miles!


9. If you want to use anti-seize on the new plugs, that's never a bad idea. Just be ABSOLUTELY sure to get none on or anywhere near the plug tip. I like to leave about a quarter to half an inch of bare metal near the tip of the plug so there's no chance of any getting near it.

10. Install everything in reverse order and you're good to go!
 

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Discussion Starter #4
Hi, great DIY! Mind if I ask why you did this? How many miles did you have? Did your MPG go up? Thanks!
I was at about 86k miles when I changed them out. They could have lasted another 10k most likely, maybe even all the way to 100k, but I was doing my wife's Mazda 3's plugs and figured I'd get them both done.

MPG hasn't changed that I can tell, but I'm in tons of traffic every day so there's only so much they would change anyways. Definitely feels a little smoother at idle though.
 

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DO NOT overtorque or cross-thread those plugs! It's an aluminum head and pretty easy to hose the threads. If you do you're screwed and will not like the amount of work required to pull the head and have a timesert or similar put in. IMHO for removal break it free and then use your FINGERS to spin it out so as to avoid the risk of placing lateral loads on the threads as the plug comes out of the hole. You MUST start the new plug by HAND, be gentle, make CERTAIN it is in the hole cleanly, and the threads engaged before anything more than very light finger pressure is used on a bare socket extension. You can and should run it all the way down until the gasket makes contact using nothing more than two fingers on the extension. A torque wrench is IMHO NOT optional for tightening either.
 

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I remember someone having issues when they didn't go OEM when they did their plug change on their 3rd gen... others had no issues, but I noticed it was only some '14 people that had issues where generally the '15 guys had none. Of course, trying to search, I couldn't find the post, and am unsure of why it seemed to be only some of the people with '14s. I just wanted to point it out incase someone has a '14 and it seems to reduce performance with the plugs listed, they should try OEM.
 

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Just as a heads up ... and I could be wrong...but if you have a OVT tune.. i remember reading somewhere that you need to tell Mat or get a new log if you change out your plugs? Maybe thats if you go aftermarket? I wish I could remember where I read that.
 

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I have an OVT tune and replaced my plugs with aftermarket, no adjustments required and with no adverse impact. I've got close to 40,000 miles on the replacements at this point and in another 10-20,000 I'll replace them again, since the aftermarket ones are reasonably-priced and a 50k interval isn't excessive.
 

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So my new to me 16' sport just hit 46K and the idle has been a little rough until it warms up. Ended up trying the denso TT plugs and switched back in less than a week. With the denso plugs the idle was much improved, however the car had no low or mid range. Felt very bogged down and was less eager to kick down and go. The denso plugs did seem to be a little more stable at the top end but i don't typically run around that high in the revs.

Went to my local Mazda dealer and they wanted $40 each for plugs. Figure I'll get another year out of the original plugs before dropping the coin on the new oems. I did find them online for $25 each and that is about as low as I've seen oem plugs priced.

So for future, go with oem plugs is my recommendation.
 

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Discussion Starter #11
So my new to me 16' sport just hit 46K and the idle has been a little rough until it warms up. Ended up trying the denso TT plugs and switched back in less than a week. With the denso plugs the idle was much improved, however the car had no low or mid range. Felt very bogged down and was less eager to kick down and go. The denso plugs did seem to be a little more stable at the top end but i don't typically run around that high in the revs.

Went to my local Mazda dealer and they wanted $40 each for plugs. Figure I'll get another year out of the original plugs before dropping the coin on the new oems. I did find them online for $25 each and that is about as low as I've seen oem plugs priced.

So for future, go with oem plugs is my recommendation.
Thank you for replying to this thread - I had forgot I listed those Densos.

Agreed. Those Densos are not worth saving the cash. I've "only" put about 25k on them and I'll be replacing them (hopefully) this weekend with some Boche. We'll see how it goes - I refuse to pay that ludicrous amount for OEM.
 

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I think you can find oem equivalent NGK just by looking up the oem part number. They're marked exactly the same, and the plugs the dealer parts guy showed me are actually labelled NGK and came out of an NGK pack. When it is due I'll likely do just that. In the past on my Volvo's I've used tasca parts online, and they also sell mazda parts. Pretty good deals to be had, I'd just make sure you spend enough to justify the shipping because they typically charge a bit for that.
 

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So my new to me 16' sport just hit 46K and the idle has been a little rough until it warms up. Ended up trying the denso TT plugs and switched back in less than a week. With the denso plugs the idle was much improved, however the car had no low or mid range. Felt very bogged down and was less eager to kick down and go. The denso plugs did seem to be a little more stable at the top end but i don't typically run around that high in the revs.

Went to my local Mazda dealer and they wanted $40 each for plugs. Figure I'll get another year out of the original plugs before dropping the coin on the new oems. I did find them online for $25 each and that is about as low as I've seen oem plugs priced.

So for future, go with oem plugs is my recommendation.
My new to me '14 had 97k on the original Mazda plugs. Start up and initial idle were a little rough so I went about sourcing a replacement. I did buy the Denso 4711 Iridium plugs for about $24 total.

After going through a tank of gas with the new plugs installed I have to agree with Ih8spdbumps. Start up and hot/cold idle are much better and smoother, but low and mid-range torque have suffered. Getting caught below 2500rpm or so in 2nd gear and needing to move quickly results in a delayed response - almost bogging. Once it gets above 3500rpm it'll hit the powerband and scoot like I need it to. I might have to look at going back to OEM sooner than later, cause this is annoying!

Also, a 14mm (or 9/16") thin wall spark plug socket is a necessity! I've got a spark plug socket for everything else so I was a little surprised this was something new. Overall very easy to access and replace the plugs though.
 

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Discussion Starter #14
My new to me '14 had 97k on the original Mazda plugs. Start up and initial idle were a little rough so I went about sourcing a replacement. I did buy the Denso 4711 Iridium plugs for about $24 total.

After going through a tank of gas with the new plugs installed I have to agree with Ih8spdbumps. Start up and hot/cold idle are much better and smoother, but low and mid-range torque have suffered. Getting caught below 2500rpm or so in 2nd gear and needing to move quickly results in a delayed response - almost bogging. Once it gets above 3500rpm it'll hit the powerband and scoot like I need it to. I might have to look at going back to OEM sooner than later, cause this is annoying!

Also, a 14mm (or 9/16") thin wall spark plug socket is a necessity! I've got a spark plug socket for everything else so I was a little surprised this was something new. Overall very easy to access and replace the plugs though.
I swapped over to the Bosch ones I listed in the OP about 15k miles ago and not looking back. Idle at times is a tiny bit rough, but I vaguely remember the OEM ones were the same. Low/Mid torque is back to normal and the smile has returned when getting on highways with my little 4-banger. :)
 

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I swapped over to the Bosch ones I listed in the OP about 15k miles ago and not looking back. Idle at times is a tiny bit rough, but I vaguely remember the OEM ones were the same. Low/Mid torque is back to normal and the smile has returned when getting on highways with my little 4-banger. :)
Again, totally agree with the observations on this board. I swapped the Denso's out for the Bosch Iridium and am glad I did. The low and mid-range torque has recovered and this thing is running pretty smooth for me now. I was able to get the Bosch plugs at Advance Auto for $19.70 total including tax so I'm pretty happy. I might look at OEM the next time around, but for $20 I'm satisfied with how the car runs.

On a related note, if somebody needs a 400-mile old set of Denso 4711 plugs cheap, I'm your man!
 

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Again, totally agree with the observations on this board. I swapped the Denso's out for the Bosch Iridium and am glad I did. The low and mid-range torque has recovered and this thing is running pretty smooth for me now. I was able to get the Bosch plugs at Advance Auto for $19.70 total including tax so I'm pretty happy. I might look at OEM the next time around, but for $20 I'm satisfied with how the car runs.
+1 Bosch.... I have tried Denso, NGK and then Bosch - huge difference.... 90K on the car and it drives peppier and smoother than with Denso's or NGK's.
I purchased from Autozone, cost me 12 dollars after their 20% ship to home discount and 10 dollar rebate.
 

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Discussion Starter #18
Part number for bosch? or is it just getting bosch brand when at the auto parts store?
Part numbers are on the original post of the thread :)

Boche: 9621
Denso: 4711
NGK: 94124
 

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I have heard that the originally-fitted brand has noticeably better results. These are apparently either Denso or NGK? Does anyone know which is the OEM brand?
So far as I remember, the OEMs were NKG - but don't quote me on that.

I had originally got the Denso from Amazon and didn't like them at all. Note that I got them for a great price on Amazon, which may have played a role, but I didn't keep them in there for very long (maybe a month) before swapping in Boche - noticeably smoother idle and acceleration was like new.

Could the Denso I got have been a bad batch, of course, but I won't go back unless the Boche start to feel the same way.
 
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