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Rough Idle and Misfire on 2016 Crosstrek 2.0i

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Mike T. · rep 0 · May 25, 2:14 AM · 1 replies · solved · Question

Vehicle: 2016 Subaru Crosstrek 2.0i Premium

My 2016 Subaru Crosstrek 2.0i Premium, with about 90,000 miles, has recently started developing a rough idle. It's most noticeable when I'm stopped at a light or in traffic. Sometimes, it feels like it's going to stall, and the engine shakes a bit more than usual. The Check Engine Light came on yesterday and started flashing briefly before staying solid.

I pulled the code with a basic OBD scanner, and it's showing P0303, indicating a misfire in cylinder 3. What could be causing this? I've had the spark plugs replaced around 60k miles, but maybe it's time again?

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 2:14 AM

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A rough idle accompanied by a P0303 (Cylinder 3 Misfire Detected) code on your 2016 Crosstrek points directly to an issue affecting the combustion process in that specific cylinder. A flashing CEL indicates a severe misfire that could be damaging your catalytic converter, so it's important to address this promptly.

Here are the most common causes for a single-cylinder misfire:

  1. Ignition System:
    • Spark Plug: Even if replaced at 60k, a faulty or fouled spark plug in cylinder 3 could be the culprit. Inspect it for wear, cracks, or oil/carbon buildup.
    • Ignition Coil: The ignition coil for cylinder 3 might be failing. Coils can degrade over time, leading to intermittent or complete misfires. You can often swap coils between cylinders to see if the misfire moves.
    • Spark Plug Wire (if applicable): Less common on modern engines, but a damaged wire could cause issues.
  2. Fuel System:
    • Fuel Injector: A clogged, dirty, or failing fuel injector for cylinder 3 could be delivering insufficient or no fuel, leading to a lean misfire.
  3. Compression Issues:
    • Low Compression: While less common for a sudden onset, issues like a leaky valve, worn piston rings, or a blown head gasket (though less frequent on these newer FB engines for external leaks) could cause low compression in cylinder 3, leading to a misfire.

What to do:

  • Inspect Spark Plug: Start by removing and inspecting the spark plug from cylinder 3. Look for signs of fouling, damage, or improper gap.
  • Swap Ignition Coil: If the spark plug looks good, try swapping the ignition coil from cylinder 3 with a coil from a different cylinder (e.g., cylinder 1). Clear the code and drive. If the misfire code changes to P0301, you've identified a faulty coil.
  • Fuel Injector Test: A mechanic can perform a fuel injector balance test or remove and visually inspect the injector for clogs.
  • Compression Test: If the above steps don't resolve the issue, a compression and leak-down test will determine if there's an internal engine problem.

When to see a shop: Since you have a flashing CEL, you should have this diagnosed by a professional as soon as possible. Driving with a persistent misfire can lead to significant damage to your catalytic converter, which is an expensive repair. A shop can perform comprehensive diagnostics to pinpoint the exact cause efficiently.

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