Replacing Spark Plugs on a Mazda 3
Mazda OEM spark plugs are iridium plugs from NGK. Iridium's have a very long service life, and perform fantastically.
There's no point in buying any different types of spark plug, the OEM's are the best suited for the motor, and other plugs are are very likely to cause a decrease in combustion, performance, and fuel mileage.
Remove Engine Cover
The cover is very simple to remove, simply pull upwards and it will pop out of the 3 clips that hold it in.
Remove Coil Packs
First remove the plastic clip that is keeping you from removing the left most coil pack's retaining stud. I used a pair of pliers to pull up on the clip while using a pair of snap ring pliers to disengage the little teeth on the clip. They are a little weird, but once I had the right set of tools, it pulled right off without damaging the clip.
Each coil has one stud keeping it in place. Using a deep 8mm socket loosen each of the four studs and remove them.
Once the stud is removed, gently pull each coil pack and attached rubber boot out from the top of the cylinder head. Set aside, there is no need to detach them from the electrical connectors.
Remove Old Spark Plugs
Using a 5/8" Spark plug socket, loosen and remove each of the spark plugs.
Install New Spark Plugs
Install each of the 4 spark plugs.
My favorite trick to keep from cross threading a plug into the head, is to use a rubber or silicone hose pushed onto the end of the spark plug. The hose will allow you to more easily feel the correct thread engagement of the spark plug then using a wrench would. And, ideally the hose won't grip the plug as strongly, so if you do mis-thread it, you won't be able to tighten it enough to cause damage to the cylinder head.
Install Spark Plug Coils
Press each coil down firmly into the spark plug hole, making sure the electrode is fully seated onto the plug. Re-install the 8mm stud, and hand tighten.
Install Engine Cover
Slap the engine cover back on.
Test It Out
Jump in and start it up. The engine should run at least as smooth as it did before you changed the spark plugs. If there are any issues, check the electrical connections on the coils and make sure they are all properly seated onto the spark plugs.