Spark plugs produce a spark that ignites the gas in a motor chamber. Without a spark to properly ignite the gas, a gas-powered motor cannot run. Knowing when to replace a spark plug is an important part of any gas-powered engine maintenance plan.

Step 1: To check a spark plug on a cool engine, pull off the spark plug wire and use a wrench or deep well socket to remove the spark plug.

Step 2: The central electrode runs through the center of the plug and is seen as a metal tip surrounded by insulating porcelain at the base of the plug. Electric current is carried through the central electrode to the grounding electrode, which is the L-shaped piece of metal located at the bottom of the plug. Use a wire brush or soft cloth, clean away oil or carbon deposits from the electrodes. Replace the plug if either electrode appears damaged or worn.

Step 3: Replace the plug if any of the porcelain appears stressed or broken.

Step 4: Use an Ohm meter to test the spark plug. Hold one probe on the bottom metal ridge at the top of the spark plug and touch the other probe to the metal nub of the central electrode at the bottom of the plug. The Ohm meter should register some level of resistance. Keeping one probe on the top of the plug, place the second probe on the metal-threaded portion of the plug. If the Ohm meter registers any resistance, the plug is bad and needs to be replaced.

Step 4: When replacing any spark plug, follow the engine manufacturer’s recommendation.

Step 5: The letters and numbers that make up a spark plug’s part number indicate thread size, length and other important attributes. The number in the middle of the part number indicates the plug’s heat rating. Hotter plugs release less heat into the engine block, corrode less and last longer. Smaller numbers indicate hotter plugs. When an exact plug match is not available, using a plug that is one number lower than the recommended plug is a safe option.

Step 6: Resistor plugs are indicated by an “R” in their part number. Resistors suppress ignition noise when a plug is sparked. Resistor plugs are only necessary in engines with electronic ignitions or computerized controls. Choose a non-resistor plug for most small engines.

— Have a home improvement question for the Fix-It Chick? Email it to Linda Cottin at features@ljworld.com.