Worn or dirty plugs may give satisfactory operation at idling speed, but under operating conditions they frequently fail. Faulty plugs are indicated in a number of ways: poor fuel economy, power loss, loss of speed, hard starting and general poor engine performance.

Spark plugs may also fail due to carbon fouling, excessive gap, or broken insulator.

Fouled plugs are identified by black carbon deposits. the black deposits are usually the result of slow-speed driving and short runs where sufficient engine temperature is seldom reached. Worn pistons, rings, faulty ignition, over-rich air/fuel mixture and spark plugs which are too cold will also result in carbon deposits.

Excessive gap wear, on plugs of low mileage usually indicates the engine is operating at high speeds or loads that are consistently greater than normal or that a plug which is too hot is being used. Electrode wear may also be the result of plug overheating due to insufficient spark plug torque or too much spark plug torque. An excessively lean air/fuel mixture will also result in excessive electrode wear.

Broken insulators are usually the result of improper installation or carelessness when regapping the plug. broken upper insulators usually result from a poor fitting wrench or an outside impact. The cracked insulator may not show up right away, but will as soon as oil or moisture penetrates the crack. The crack may not be visible.

Broken lower insulators usually result from carelessness when regapping and generally are visible. This type of break may result from the plug operating too hot, which may happen in periods of high-speed operation or under heavy loads. When regapping a spark plug, always make the gap adjustment by bending the ground (side) electrode. Spark plugs with broken insulators should always be replaced.