Getting Started. Fouled, damaged, or worn out spark plugs can lead to a variety of engine problems, such as misfires, hard starts, increased emissions, and poor fuel economy. For most vehicles, you should replace your spark plugs every 30,000 miles (consult your owner's manual for your specific interval). It is also recommended that you replace your spark plug wires at the same time, or you may still experience misfires or other problems.