2021 CT5 V6 Check Engine Light & Rough Idle - P0304

ClearTheCode admin answer:

1. Verify the Code and Clear: First, confirm the P0304 code is persistent. Sometimes a single misfire can trigger it. Clear the code and see if it returns immediately. If it does, proceed with further diagnostics.
2. Inspect Ignition Components: A misfire on a specific cylinder (P0304 for cylinder 4) most commonly points to an issue with the ignition system.
   • Spark Plug: Remove and inspect the spark plug for cylinder 4. Look for signs of fouling, wear, or damage. Compare it to a plug from a known good cylinder if possible.
   • Ignition Coil: The ignition coil for cylinder 4 could be failing. You can try swapping the coil from cylinder 4 with a coil from a different cylinder (e.g., cylinder 2 or 6). If the misfire code then moves to the new cylinder (e.g., P0302 or P0306), you've identified a faulty coil.
3. Check Fuel Delivery: If ignition components appear fine, the next area to investigate is fuel delivery.
   • Fuel Injector: A clogged or faulty fuel injector for cylinder 4 could cause a lean misfire. This is harder to diagnose without specialized tools, but sometimes a fuel system cleaner can help if it's a minor clog.
4. Engine Compression: Less common but possible, low compression in cylinder 4 due to a valve issue or piston ring problem could cause a misfire. This requires a compression test.
5. When to See a Shop: If you've tried swapping coils and inspecting the spark plug without success, or if the problem persists and you're not comfortable with further diagnostics, it's best to take your CT5 to a qualified Cadillac service center. They have advanced diagnostic tools to pinpoint the exact cause, including injector testing and compression checks. Driving with a persistent misfire can damage the catalytic converter over time.