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Rough Idle & Misfire Codes on 2020 Forester Sport - Spark Plugs?

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Mike T. · rep 0 · May 22, 3:19 AM · 1 replies · solved · Question

Vehicle: 2020 Subaru Forester Sport

Hi all,

My 2020 Subaru Forester Sport (2.5L engine) has recently developed a rough idle, especially when cold. It feels like a slight shudder, and sometimes I notice a slight hesitation during acceleration. The check engine light came on briefly a few times and then went off, but I managed to pull some codes before it cleared.

The codes I got were P0301 and P0303, indicating misfires on cylinders 1 and 3. The car has about 65,000 miles on it. Could this be related to the spark plugs? What's the recommended interval for spark plug replacement on these newer Foresters? Is this a DIY job or best left to a professional given the Boxer engine layout?

✓ Accepted solution

ClearTheCode Admin

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

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ClearTheCode admin answer:

Given the symptoms and the OBD-II codes (P0301, P0303), it's highly probable that your spark plugs are a primary suspect, especially with 65,000 miles on your 2020 Forester Sport. Misfire codes directly point to issues with combustion in specific cylinders.

1. Spark Plug Replacement Interval:

  • For most 2.5L naturally aspirated Subaru engines (like in your Forester), the recommended spark plug replacement interval is typically every 60,000 miles. You are right in that sweet spot where they are due or overdue.

2. Common Causes of Misfires (P030X codes):

  • Worn Spark Plugs: The most common cause at your mileage. Electrodes wear down, requiring more voltage to spark, leading to weak or no spark.
  • Ignition Coils: A failing ignition coil for a specific cylinder can also cause misfires. Since you have misfires on two cylinders, it's less likely to be two coils failing simultaneously, but not impossible.
  • Fuel Injectors: Clogged or faulty fuel injectors can lead to an improper air/fuel mixture.
  • Vacuum Leaks: Can cause a lean condition and misfires.
  • Compression Issues: Less common but possible, especially if other causes are ruled out.

3. DIY Feasibility for Boxer Engine:

  • Replacing spark plugs on a Subaru Boxer engine can be more challenging than on an inline engine due to the horizontally opposed cylinders. Access to the plugs is tight, requiring specific tools and patience.
  • Tools Needed: You'll typically need a universal joint or wobble extension for your ratchet, a spark plug socket (often a thin-wall 14mm or 16mm, check your manual), and a torque wrench.
  • Procedure: It often involves removing the airbox on one side and potentially the battery/washer fluid reservoir on the other for better access. Take care not to overtighten the new plugs or damage the ignition coil boots.

4. When to See a Shop:

  • If you're not comfortable working in tight spaces or lack the specific tools.
  • If, after replacing the spark plugs, the misfires persist or new codes appear. This would indicate a deeper issue beyond just the plugs, such as ignition coils, fuel delivery, or even a vacuum leak.
  • A professional can perform a full diagnostic, including checking ignition coil resistance, fuel pressure, and compression tests, to pinpoint the exact cause if plugs don't resolve it.
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