The spark plugs of a BMW X1 should generally be replaced every 30,000 to 40,000 miles, or every two years, whichever comes first. This interval is based on the manufacturer's recommendations and is designed to ensure optimal engine performance and fuel efficiency.

The reason for this regular maintenance is that spark plugs play a crucial role in the combustion process within the engine. Over time, the electrodes of the spark plug can become worn, eroded, or coated with deposits, which can lead to a weakened spark. This weakened spark can result in misfires, reduced power, and increased fuel consumption. Additionally, worn-out spark plugs can contribute to engine damage if not replaced in a timely manner.

According to a study by the Car Care Council, spark plug failure can lead to a decrease in fuel efficiency of up to 30%. Furthermore, the National Institute for Automotive Service Excellence (ASE) advises that spark plug maintenance is one of the simplest and most cost-effective ways to ensure the longevity of an automotive engine. By adhering to the recommended replacement schedule, BMW X1 owners can prevent potential engine issues and maintain the vehicle's performance and reliability.