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2020 Sierra 1500 AT4 - P0304 Misfire Code & Rough Idle

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Mike T. · rep 0 · May 24, 1:29 PM · 1 replies · solved · Question

Vehicle: 2020 GMC Sierra 1500 AT4

Hey everyone,

My 2020 GMC Sierra 1500 AT4, which has the 6.2L V8 engine, recently started acting up. About a week ago, I noticed a slight rough idle, and then the Check Engine Light came on solid. I hooked up my OBD-II scanner, and it pulled a P0304 code, indicating a misfire on Cylinder 4.

The truck feels a bit sluggish, especially under acceleration, and the idle is definitely not as smooth as it used to be. I've got about 65,000 miles on it, and I've kept up with all the regular maintenance.

Has anyone else experienced this specific code or misfire issues with their Sierra? What were the common culprits you found? I'm trying to figure out if this is something I can tackle myself or if it's best to take it straight to the dealership. Any advice on where to start troubleshooting would be greatly appreciated!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 24, 1:29 PM

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ClearTheCode admin answer:

A P0304 code, indicating a misfire on Cylinder 4, is a common diagnostic trouble code. It signals reduced engine performance and can harm your catalytic converter if ignored.

Here are common causes and diagnostic steps:

  1. Spark Plugs/Ignition System:

    • Cause: A worn spark plug, faulty ignition coil pack for cylinder 4, or damaged spark plug wire. Given your mileage, plugs are a good starting point.
    • Diagnosis: Inspect cylinder 4's spark plug for wear. Swap the cylinder 4 ignition coil with an adjacent one (e.g., cylinder 2) to see if the misfire code follows (e.g., P0302). If so, replace the coil.

  2. Fuel Delivery:

    • Cause: Clogged or faulty fuel injector for cylinder 4, or low fuel pressure. Carbon buildup on the injector tip is common.
    • Diagnosis: A professional fuel injector test is often needed. You can try a high-quality fuel system cleaner, but a truly faulty injector requires replacement.

  3. Engine Mechanical Issues:

    • Cause: Low compression in cylinder 4 from a worn piston ring, bent valve, or blown head gasket. This is less common but serious.
    • Diagnosis: A compression test or leak-down test is essential for this diagnosis, typically performed by a qualified technician.

  4. Vacuum Leaks:

    • Cause: A vacuum leak near cylinder 4 can lean the air/fuel mixture, causing a misfire.
    • Diagnosis: Visually inspect vacuum lines for cracks or disconnections.

When to See a Shop: If basic checks like spark plug inspection and coil swap don't resolve the P0304, or if you're uncomfortable with further diagnostics, it's best to consult a certified mechanic. They possess specialized tools for advanced tests (fuel pressure, injector pulse, compression) to accurately identify and repair the root cause.

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