How to Know if Lawnmower Spark Plugs Need to Be Changed

Locate the spark plug. The top of the plug will be covered by a protective rubber cover, called a boot, from which a single thick wire -- the high-tension lead -- emerges. Grip the rubber boot covering the top of the plug, and then twist and pull at the same time to release the boot and lead together. Wipe away any debris, such as dust and oily grass, collected around the plug and the recess in which it sits.

Unscrew the plug using a spark plug wrench. Apply pressure gently when starting to turn the plug; the engine block is often made of aluminum and a violent jerk of the plug may damage the threaded inset. Remove the plug.

If the plug is obviously cracked or damaged, or has been used for more than 100 hours or for an entire season, replace it.

Replace the plug if the electrode or the insulator nose -- which are the central metal rod and the surrounding ceramic ring, respectively -- are coated in a glossy black residue of soot and oil. This is caused by too much oil in the cylinder, and often results in misfiring and problems starting the mower.

Check the plug for signs of a green/yellow glaze. The plug nose should be white. This problem occurs only with leaded fuel and, since the glaze conducts electricity and short-circuits the plug, replacement is the only option.

Examine the nose and electrode for brown ash-like deposits. These resemble the clinker sometimes found in the bottom of a stove. Modern engine oils may contain additional chemicals to improve viscosity and reduce engine wear. The ash deposit is the burnt remains of the additives. The plugs cannot be cleaned and must be replaced.

Examine the central electrode and the ground electrode -- the metal bar positioned just above the central electrode -- for signs of wear and melting. It they are fused together, pitted, eroded or coated in melted debris the plugs require replacement.