Have you modified your Focus ST with more power and boost? If so a colder heat range spark plug is highly recommended. These Denso ITV22 spark plugs for your Focus ST are one heat range colder. They will directly replace the factory spark plugs with ease. A colder heat range plug reduces detonation sensitivity because it transfers heat faster keeping your combustion chamber cooler.
If you are running a Stage 2 or more aggressive set up, these plugs are highly recommended.
Recommended plug gap .029 to .031