Rough Idle & Misfire Codes (P0301, P0305) on 2017 G90 5.0L

ClearTheCode admin answer:

Misfire codes P0301 and P0305 on your 2017 Genesis G90 5.0L indicate specific cylinders (1 and 5) are not firing correctly. This is a common diagnostic issue, and fortunately, there's a systematic approach to finding the cause.

Here are the most common culprits and diagnostic steps:

1. Spark Plugs: Given your mileage (75,000 miles), spark plugs are a very strong suspect. Worn or fouled spark plugs are a primary cause of misfires. Start by inspecting and potentially replacing the spark plugs on cylinders 1 and 5.
2. Ignition Coils: A failing ignition coil for a specific cylinder will prevent spark. You can often diagnose this by swapping the ignition coil from cylinder 1 to a known good cylinder (e.g., cylinder 3) and seeing if the misfire code follows the coil (i.e., if you then get a P0303). Do the same for cylinder 5.
3. Fuel Injectors: A clogged or faulty fuel injector for cylinder 1 or 5 would prevent adequate fuel delivery. This is harder to diagnose without specialized equipment. You might be able to hear the injector clicking with a stethoscope, but flow testing is more definitive.
4. Compression Issues: Less common to suddenly affect specific cylinders without other symptoms, but low compression on a cylinder due to a valve issue or head gasket problem will cause a misfire. A compression test would confirm or rule this out.
5. Vacuum Leaks: While usually causing multiple misfires or lean codes, a significant vacuum leak near a specific cylinder could theoretically affect it.

When to See a Shop:

• If replacing spark plugs and swapping coils doesn't resolve the issue.
• If you suspect a fuel injector problem, as testing and replacement can be complex.
• If a compression test is needed, which requires specific tools and expertise.
• If the misfire persists after basic troubleshooting, as further diagnostics might involve checking wiring, ECU signals, or more in-depth engine analysis.