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Misfire on 2020 Forester - Time for new spark plugs?

0

Mark S. · rep 0 · May 22, 3:19 AM · 1 replies · solved · Question

Vehicle: 2020 Subaru Forester Sport

Hello everyone,

My 2020 Subaru Forester Sport, which just hit 65,000 miles, has recently started acting up. I've noticed a slight rough idle, especially when cold, and the check engine light came on a couple of days ago. I took it to AutoZone for a quick scan, and it pulled a P0301 code.

From what I understand, P0301 indicates a misfire in cylinder 1. Given the mileage, could this be a sign that it's time to replace the spark plugs? I haven't done them yet, and I know they're a scheduled maintenance item. Is there anything else I should be checking for before diving into spark plug replacement?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 3:19 AM

0

ClearTheCode admin answer:

A P0301 code (Cylinder 1 Misfire Detected) on your 2020 Subaru Forester at 65,000 miles is a strong indicator that spark plugs could be the culprit, but it's important to consider other possibilities as well. Subaru generally recommends spark plug replacement for the 2.5L engine around 60,000 miles, so you're right in that ballpark.

Here's a diagnostic approach:

  1. Spark Plugs (Most Likely):

    • Cause: Worn spark plugs can lead to inconsistent spark, causing misfires. Over time, the electrode gap widens, and carbon deposits can build up.
    • Action: Inspect and replace all four spark plugs. Use OEM-specified iridium spark plugs for optimal performance and longevity. Note that accessing the spark plugs on a Boxer engine can be a bit challenging due to their horizontal orientation.

  2. Ignition Coils:

    • Cause: A failing ignition coil for cylinder 1 could also cause a misfire. Coils can degrade over time, especially with heat.
    • Action: You can try swapping the ignition coil from cylinder 1 with a known good cylinder (e.g., cylinder 3). If the misfire code moves to the new cylinder (e.g., P0303), then the coil is faulty.

  3. Fuel Injector:

    • Cause: A clogged or faulty fuel injector for cylinder 1 could prevent proper fuel delivery, leading to a misfire.
    • Action: This is harder to diagnose at home. A fuel pressure test or professional injector testing might be needed. Sometimes, a fuel system cleaner can help with minor clogs.

  4. Compression Issue:

    • Cause: Less common, but a mechanical issue like a worn valve, head gasket leak, or piston ring problem in cylinder 1 could cause low compression and a misfire.
    • Action: A compression test or leak-down test would be necessary to rule this out.

When to see a shop:

  • If replacing the spark plugs and/or swapping ignition coils doesn't resolve the P0301 code.
  • If you're not comfortable with the spark plug replacement process, as it requires careful handling and proper torque.
  • If you suspect a fuel injector or a more serious internal engine issue.
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