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2016 Ford Focus ST Misfire on Cylinder 3 - P0303 Code

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Mike T. · rep 0 · May 25, 1:35 AM · 1 replies · solved · Question

Vehicle: 2016 Ford Focus ST

Hey everyone, I've got a 2016 Ford Focus ST with about 95,000 miles on it, and I'm experiencing a pretty consistent misfire. The check engine light came on, and when I scanned it, I got a P0303 code, indicating a misfire on cylinder 3.

The car feels rough at idle and sometimes stutters under acceleration, especially at lower RPMs. I haven't changed the spark plugs since I bought it used at 40k miles, so they've got about 55k on them. Could this just be old spark plugs, or should I be looking at ignition coils too?

Any advice on troubleshooting this would be great. I'm hoping it's something I can tackle myself.

✓ Accepted solution

ClearTheCode Admin

ClearTheCode Team · rep 0 · May 25, 1:35 AM

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ClearTheCode admin answer:

A P0303 code (Misfire Detected - Cylinder 3) on your 2016 Ford Focus ST at 95,000 miles strongly suggests an ignition system issue, and you're on the right track considering spark plugs and ignition coils. With a boosted engine like the EcoBoost in the ST, these components work harder and can wear out faster than in naturally aspirated engines.

Common Causes of P0303 (Cylinder 3 Misfire):

  • Worn Spark Plug: This is the most common cause. Over time, the electrode gap widens, requiring more voltage to fire, eventually leading to misfires. Your plugs are well past their typical service interval for a performance engine.
  • Faulty Ignition Coil: The ignition coil for cylinder 3 could be failing, unable to produce sufficient spark. Coils can degrade over time due to heat and vibration.
  • Fuel Injector Issues: A clogged or faulty fuel injector for cylinder 3 could lead to a lean or rich condition, causing a misfire. This is less common than ignition issues but possible.
  • Compression Issues: While less likely to be the first suspect, low compression in cylinder 3 due to valve issues or piston ring wear could cause a misfire.

Recommended Action Steps:

  1. Replace Spark Plugs: Start by replacing all four spark plugs. For the Focus ST, it's often recommended to use a colder plug (e.g., NGK LTR7IX-11 or similar) and gap them correctly (typically around 0.026-0.028 inches for stock or mild tunes). Ensure you use a torque wrench to tighten them correctly.
  2. Swap Ignition Coil: If new spark plugs don't resolve the issue, swap the ignition coil from cylinder 3 to a different cylinder (e.g., cylinder 1). Clear the code, drive the car, and re-scan. If the misfire code moves to the new cylinder (e.g., P0301), you've identified a faulty coil.
  3. Inspect Fuel Injector: If the misfire persists after addressing plugs and coils, consider inspecting or testing the fuel injector for cylinder 3. This often requires specialized tools or a shop.

When to See a Shop:

  • If replacing spark plugs and swapping coils doesn't resolve the P0303 code.
  • If you suspect a fuel injector issue or compression problem, as these require more advanced diagnostics.
  • If you're uncomfortable performing the spark plug or coil replacement yourself.
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