If a spark plug has stopped working, you may just need to clean it or regap it, rather than replace it. Many spark plugs cost under $10, but if you know how to regap a spark plug you can save yourself some time and money in the long run.
To correctly gap a spark plug, you'll need a good spark plug gap gauge. There are several different kinds available, and you can probably find many of them online or in your local auto parts store. One decent gauge is the Lisle Spark Plug Gauge, $3.93 on Amazon.com. Any spark plug gauge will work, as long as it is easy to use and accurate.
In order to regap a spark plug you first have to remove it from the engine. For cars and trucks, the procedure for removing a spark plug varies by vehicle, but be careful not to damage the plug or its threads. A spark plug wrench, or a socket wrench with the correct size spark plug socket, will help to remove the plug without damaging it. On a vehicle with multiple spark plugs, only remove one plug at a time.
For spark plugs on lawn mowers or other power equipment, you can often remove them by disconnecting the spark plug boot (the wire attached to the plug) and then unscrewing them with a spark plug wrench or socket.
On the bottom of the spark plug (the end without the threads) you'll see an L-shaped piece of metal. This is the side electrode, the one that you will adjust to set the gap. The gap is the distance between this side electrode and the center electrode, the metal piece almost touching the side electrode.
You'll need to know what the gap is supposed to measure in order to regap the plug. You can often find this information in the owner's manual for the vehicle or equipment. If you don't have an owner's manual you can try to look the specs up online. For cars or trucks, you can find the spark plug gap by asking at an auto parts store.
Check the current gap by inserting your spark plug gauge in between the two electrodes. To get an accurate measurement, the gauge should fit in snugly - if it slides in too easily or if you have to force it in, then you're not accurately measuring it. Check the measurement on the gauge to see what the current gap is, and check this number against what the gap is supposed to measure. This check will tell you whether you need to make the gap larger or smaller. (For example, if the gap currently measures 0.025" and it should be 0.030" then you'll need to increase the gap.)
While you have the spark plug out (and before you regap it) is a good time to clean the spark plug. You can clean any buildup off of the electrodes with a small wire brush. If the electrodes are severely burnt, or the plug looks damaged, you'll be better off replacing it rather than trying to fix it.
To regap the spark plug, adjust the side electrode (the "L"-shaped one) either closer to or farther from the center electrode. If your spark plug gauge has a gapper tool attached to it (the Lisle gauge has a flanged hole you can use) you can use that; otherwise, a small flathead screwdriver will work.
Bend the side electrode toward the center electrode to make the gap smaller, or bend it away from the center electrode to make the gap bigger. Be very gentle when you adjust the side electrode - only bend it a little bit at a time, then check the gap again with your gauge to see if it needs to be adjusted more. Avoid touching the center electrode with your gapper or screwdriver when you are adjusting the gap size.
Once you have adjusted the gap to where it should be, double check your measurement, then you can reinsert the spark plug and reattach the spark plug wire or boot. Tighten the spark plug securely but try not to over-tighten it. If your engine has a recommended torque value for how to tight the plug should be, use a torque wrench to get the tightness as exact as possible. (For smaller engines, getting the torque exactly right may not be as important as it is for larger engines.) Then test your engine to make sure it works and the spark plug fires.
(Originally posted on Helium.com, May 2012)