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2017 Impreza Rough Idle and Misfire Codes P0301, P0303

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Mike T. · rep 0 · May 22, 3:13 AM · 1 replies · solved · Question

Vehicle: 2017 Subaru Impreza 2.0i Sport

Hey everyone,

My 2017 Subaru Impreza 2.0i Sport is giving me some headaches. Lately, it's developed a really rough idle, especially when cold, and sometimes even when warmed up. The Check Engine Light came on, and when I scanned it, I got P0301 and P0303, indicating misfires on cylinders 1 and 3.

I replaced the spark plugs about 15,000 miles ago, so they shouldn't be completely shot, but I'm wondering if it could be ignition coils or maybe even fuel injectors. The car feels a bit sluggish under acceleration too, and the fuel economy seems to have taken a hit.

Has anyone else with a 2017 Impreza experienced these specific misfire codes? What was the culprit in your case? I'm trying to narrow down the possibilities before I start throwing parts at it.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 22, 3:13 AM

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ClearTheCode admin answer:

Rough idle and misfire codes P0301 and P0303 on your 2017 Subaru Impreza 2.0i Sport are common symptoms that point to issues affecting combustion in specific cylinders. Since you've already addressed spark plugs relatively recently, we can look at other common culprits.

Here's a breakdown of potential causes and diagnostic steps:

  1. Ignition Coils: A failing ignition coil for cylinder 1 or 3 is a very common cause of misfires. Coils can degrade over time, especially with heat cycles.
    • Action: You can swap the ignition coils from cylinder 1 and 3 with known good cylinders (e.g., 2 and 4). If the misfire codes move to the new cylinders (e.g., P0302, P0304), you've identified faulty coils.
  2. Fuel Injectors: Clogged or faulty fuel injectors can lead to an insufficient fuel supply to a cylinder, causing a misfire. This is also a common issue.
    • Action: Similar to coils, you can swap injectors between cylinders to see if the misfire follows. A professional can also perform a fuel injector balance test.
  3. Vacuum Leaks: An unmetered air leak in the intake manifold or vacuum lines can lean out the air-fuel mixture, leading to misfires, especially at idle.
    • Action: Visually inspect all vacuum lines and intake manifold gaskets for cracks or signs of leaks. You can also use a smoke machine (professional tool) or spray carb cleaner around suspected areas (with caution) to see if engine RPM changes.
  4. Compression Issues: While less common for multiple cylinders simultaneously without other symptoms, low compression in cylinders 1 and 3 would definitely cause misfires.
    • Action: A compression test is the definitive way to check this. This requires specialized tools.
  5. PCV Valve: A clogged or faulty PCV (Positive Crankcase Ventilation) valve can sometimes contribute to rough idle and misfires by affecting crankcase pressure and vacuum.
    • Action: Inspect or replace the PCV valve, which is a relatively inexpensive part.

When to see a shop: If swapping coils and injectors doesn't resolve the issue, or if you're uncomfortable performing these diagnostics, it's best to take your Impreza to a qualified mechanic. They have advanced diagnostic tools to perform fuel pressure tests, compression tests, and smoke tests to accurately pinpoint the root cause of the misfires without unnecessary part replacement. Addressing misfires promptly is important to prevent potential damage to the catalytic converter.

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