A spray can of WD-40.
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How To use WD-40 To Easily Remove Spark Plugs
One of the best ways to loosen a stuck spark plug is by spraying some WD-40 on it. It will help you loosen a corroded or rusted spark plug that's locked in its socket.
Spray WD-40 around the base so it can penetrate the socket. If there's a lot of buildup, you can spray the WD-40 in this area before wiping away the grime.
Let the solvent sit for a few minutes and then try loosening it with a wrench. You may have to repeat this process a few times or allow the WD-40 to sit overnight to loosen it.
Once removed, you can spray WD-40 directly into the spark plug hole to lubricate it, reducing the buildup of carbon and corrosion, making the next removal easier.