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2020 Ram 1500 Misfire on Cylinder 3 - P0303 Code & Rough Idle

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Chris P. · rep 0 · May 24, 1:27 PM · 1 replies · solved · Question

Vehicle: 2020 Ram 1500 Laramie 5.7L Hemi

Hey everyone, hoping someone can help me out here. My 2020 Ram 1500 Laramie, 5.7L Hemi, just started acting up this morning. I've got about 75,000 miles on it.

When I started it up, it had a really rough idle, almost like it was going to stall. The check engine light came on almost immediately. I pulled out my OBD-II scanner and it's throwing a P0303 code, which I know means a misfire on cylinder 3.

I haven't done any major work recently, just routine oil changes. I filled up with gas a couple of days ago, but it was at my usual station. Any ideas on where I should start looking? Could it be something simple like a spark plug, or something more serious?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 24, 1:27 PM

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ClearTheCode admin answer:

The P0303 code indicates a misfire detected in cylinder 3. This means the engine's computer has detected an irregular combustion event in that specific cylinder. While it can be frustrating, there are several common causes you can investigate:

  1. Ignition System Issues:

    • Faulty Spark Plug: The most common culprit. Spark plugs wear out over time, leading to weak or no spark.
    • Bad Ignition Coil: Each cylinder typically has its own ignition coil. If the coil for cylinder 3 is failing, it won't provide adequate spark.
    • Damaged Spark Plug Wire (if applicable): Though less common on modern Hemi engines which often use coil-on-plug, a damaged wire could cause issues.

  2. Fuel System Issues:

    • Clogged Fuel Injector: If the fuel injector for cylinder 3 is clogged or malfunctioning, it won't deliver the correct amount of fuel, leading to a lean misfire.
    • Low Fuel Pressure: Less likely to be cylinder-specific, but overall low fuel pressure can cause misfires.

  3. Engine Mechanical Issues:

    • Low Compression: This is a more serious issue, indicating problems like worn piston rings, damaged valves, or a blown head gasket.
    • Vacuum Leak: A vacuum leak near cylinder 3 could cause it to run lean.

Diagnostic Steps You Can Take:

  • Swap Components:
    • Spark Plug: Move the cylinder 3 spark plug to cylinder 1 (or another cylinder). Clear the code and drive. If the misfire moves to the new cylinder (e.g., P0301), the plug is bad.
    • Ignition Coil: Do the same with the ignition coil. Swap cylinder 3's coil with another cylinder's. If the misfire follows, the coil is faulty.
  • Inspect Wiring: Check the wiring harness leading to the coil and injector for any visible damage.
  • Listen for Vacuum Leaks: Use a stethoscope or listen carefully for a hissing sound around the intake manifold.

When to See a Shop:

If swapping components doesn't isolate the issue, or if you suspect fuel injector problems, low compression, or a significant vacuum leak, it's best to take your Ram to a qualified mechanic. They have specialized tools for fuel pressure testing, compression testing, and advanced diagnostics to pinpoint the exact cause without guesswork, preventing potential further damage.

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