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Rough Idle and Misfire on 2019 WRX - P0301 Code

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

Vehicle: 2019 Subaru WRX Base

What's up, WRX fam?

My 2019 WRX (manual, stock engine) has developed a really rough idle lately, and I'm feeling some hesitation under acceleration. The Check Engine Light came on, and when I pulled the codes, I got a P0301 – Misfire Detected Cylinder 1. It's got about 45,000 miles on it.

I'm thinking spark plugs or ignition coil, but I wanted to get some opinions before I start tearing into it. Could it be something else common on these FA20DIT engines? Any specific things I should look out for or test first? I'm hoping it's not something more serious.

✓ Accepted solution

ClearTheCode Admin

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

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ClearTheCode admin answer:

A P0301 code on your 2019 Subaru WRX (FA20DIT engine) indicates a misfire specifically on Cylinder 1. This is a common diagnostic code, and while it points to a specific cylinder, the cause can vary. It's good you're thinking about spark plugs and ignition coils, as these are often the culprits.

  1. Spark Plugs: At 45,000 miles, your spark plugs are nearing or at their recommended replacement interval for performance engines like the FA20DIT. A worn or fouled spark plug in Cylinder 1 is a very common cause of a P0301. Inspect the plug for wear, carbon buildup, or damage.
  2. Ignition Coil: The ignition coil for Cylinder 1 could be failing. You can often diagnose this by swapping the coil from Cylinder 1 with one from another cylinder (e.g., Cylinder 3). If the misfire code then changes to P0303, you've identified a faulty coil.
  3. Fuel Injector: A clogged or failing fuel injector for Cylinder 1 can cause it to run lean or rich, leading to a misfire. This is less common than plugs or coils but is a possibility. Fuel injector cleaner might help if it's a minor clog, but a failing injector would need replacement.
  4. Compression Issues: While less likely without other symptoms (like excessive smoke or oil consumption), low compression in Cylinder 1 due to valve issues or piston ring wear can cause a misfire. A compression test would confirm or rule this out.
  5. Vacuum Leaks: A vacuum leak near Cylinder 1 could cause it to run lean and misfire. Listen for hissing sounds around the intake manifold.

When to See a Shop:

  • If swapping coils doesn't move the misfire, and replacing the spark plug doesn't resolve the issue, it's time to take your WRX to a professional. They can perform a fuel injector balance test, compression test, and more advanced diagnostics to pinpoint the exact cause.
  • Ignoring a persistent misfire can lead to more severe engine damage, including catalytic converter failure due to unburnt fuel entering the exhaust system. Address it promptly to maintain your WRX's performance and longevity.
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