A good spark plug has clean electrodes, the correct gap, and produces a strong spark that supports smooth engine operation. A bad spark plug may be worn, fouled, cracked, or overheated, resulting in m···

Most modern spark plugs should be replaced every 30,000 to 100,000 miles (48,000 to 160,000 km), depending on the spark plug material and the vehicle manufacturer's maintenance schedule.

Cleaning a spark plug involves removing the plug, inspecting its condition, cleaning away carbon and oil deposits with solvent and a wire brush, checking the gap, and reinstalling it correctly.

A complete spark plug test involves visual inspection, gap measurement, spark quality evaluation, resistance testing, and electrode wear assessment. The best spark plug will display clean electrodes.

The easiest way to identify a bad spark plug is to look for symptoms such as misfires, rough idle, hard starting, poor fuel economy, and reduced engine power. A visual inspection can reveal carbon fou···