2016 Chrysler 300C Misfire - P0304 Code and Rough Idle

ClearTheCode admin answer:

A P0304 code on your 2016 Chrysler 300C 5.7L Hemi points directly to a misfire detected in cylinder #4. This is a common diagnostic code, and several components can cause it. Here's a structured approach to troubleshooting:

1. Spark Plug and Ignition Coil: These are the most common culprits. Start by inspecting and potentially replacing the spark plug in cylinder #4. While you're there, visually inspect the ignition coil for any signs of arcing or damage. A common diagnostic step is to swap the ignition coil from cylinder #4 with a known good cylinder (e.g., #2) and see if the misfire code moves to the new cylinder. If it does, the coil is faulty.
2. Fuel Injector: A clogged or faulty fuel injector for cylinder #4 can prevent it from getting enough fuel, causing a misfire. You can test injector resistance or listen for its clicking operation with a mechanic's stethoscope. A fuel pressure test might also be warranted to rule out fuel delivery issues to the rail.
3. Compression Test: Low compression in cylinder #4 could indicate a more serious internal engine issue, such as a worn piston ring, bent valve, or faulty lifter. Given the mileage, this is a possibility with some Hemi engines. A compression test (wet and dry) will confirm the health of the cylinder.
4. Wiring Issues: Less common, but damaged wiring to the spark plug, coil, or fuel injector for cylinder #4 could also cause a misfire.

When to See a Shop: If swapping the coil and replacing the spark plug doesn't resolve the P0304 code, or if you suspect a fuel injector or internal engine problem, it's highly recommended to take your 300C to a professional mechanic. They have specialized tools for advanced diagnostics, such as scope testing for ignition patterns, fuel injector pulse testing, and precise compression/leak-down tests, which can accurately pinpoint the root cause without unnecessary parts replacement.