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Rough idle and misfires on 2019 Mazda 3 - P0301 code

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Emily N. · rep 0 · May 22, 4:51 AM · 1 replies · solved · Question

Vehicle: 2019 Mazda 3 Preferred

My 2019 Mazda 3 Preferred, with about 65,000 miles, has started acting up. I'm experiencing a really rough idle, especially when cold, and it feels like it's misfiring under light acceleration. The check engine light came on, and when I scanned it, I got a P0301 code, indicating a misfire in cylinder 1.

I'm thinking it might be spark plugs or ignition coils. I haven't changed the spark plugs yet, so they're original. Is this a common issue for these models around this mileage? What's the best way to diagnose if it's the plug or the coil pack?

Any advice on DIY replacement or what parts to look for would be greatly appreciated!

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 4:51 AM

0

ClearTheCode admin answer:

A P0301 code on your 2019 Mazda 3 at 65,000 miles, indicating a misfire in cylinder 1, is a common symptom that can often be traced back to ignition system components. Given your mileage, both spark plugs and ignition coils are prime suspects.

Here's a diagnostic and maintenance approach:

  1. Spark Plugs: Mazda typically recommends spark plug replacement around 75,000 miles for SKYACTIV engines, but they can wear out sooner, especially if there are other contributing factors. Worn plugs can lead to weak spark and misfires.
  2. Ignition Coils: A failing ignition coil is another very common cause of a P030X misfire code. The coil provides the high voltage to the spark plug.

DIY Diagnosis Steps:

  • Swap Coil Packs: The easiest way to diagnose if it's the coil is to swap the ignition coil from cylinder 1 to a different cylinder (e.g., cylinder 2 or 3). Clear the code, drive the car, and see if the misfire code changes to P0302 or P0303. If it does, the coil is faulty.
  • Inspect Spark Plug: If swapping the coil doesn't move the code, remove the spark plug from cylinder 1. Look for signs of fouling (oil, carbon buildup), damage to the electrode, or excessive gap. Compare it to a new plug or one from a healthy cylinder.

Replacement Tips:

  • Spark Plugs: Use only OEM equivalent iridium spark plugs (e.g., NGK or Denso specified for your engine). Ensure proper gapping if required, though most come pre-gapped. Apply a small amount of anti-seize to the threads and dielectric grease to the ceramic portion.
  • Ignition Coils: If replacing a coil, it's often recommended to replace all of them, especially if they are original and at similar mileage, as others may soon follow. However, replacing just the faulty one is also an option.

When to See a Shop:

  • If the misfire persists after replacing the spark plug and coil for cylinder 1.
  • If you're uncomfortable performing the diagnosis or replacement yourself.
  • If the issue is accompanied by other symptoms like excessive smoke, coolant loss, or unusual engine noises, which could indicate more serious engine problems like a fuel injector issue or compression loss.
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