2004 Ford Focus, 2 Liter Zetec engine.
0. Situation: Car initially running well but want to improve mileage, so I installed premium plugs.
- Ran roughly on test run, so headed home. Check Engine light came on before fully warmed up.
- Found and corrected loose plug wire.
- Engine now purrs like a kitten, but Check Engine light stays on.
What should I do now?