2003 Ford Focus 1.8L Repeated P0301 Misfire Code with No Visible Symptoms
2003 Ford Focus equipped with a 1.8L petrol engine, having accumulated over 171,000 miles. The vehicle consistently displays a 'Check Engine' light and logs fault code P0301 — indicating a misfire in cylinder #1. Despite no noticeable performance issues or rough idling, the problem recurs after replacing both the spark plugs and coil pack within the same day. What other potential causes could be responsible for this recurring misfire?
Have you confirmed that the spark plug is actually receiving a spark? Check the plug appearance — look for signs of wetness, black soot, or dry/tan coloration. Also, verify fuel injector operation using a noid light; this is a simple and effective DIY test. As a next step, try swapping the fuel injectors between cylinder 1 and 3 — if the same P0301 code appears after the swap, it may point to an issue with the engine's compression or internal components. A full compression test would be the final diagnostic step to identify any underlying mechanical problems.